diff --git a/apps/design_system_gallery/lib/app/gallery_app.directories.g.dart b/apps/design_system_gallery/lib/app/gallery_app.directories.g.dart
index 8d13c1b0..a9a5b671 100644
--- a/apps/design_system_gallery/lib/app/gallery_app.directories.g.dart
+++ b/apps/design_system_gallery/lib/app/gallery_app.directories.g.dart
@@ -78,6 +78,8 @@ import 'package:design_system_gallery/components/controls/stream_video_play_indi
     as _design_system_gallery_components_controls_stream_video_play_indicator;
 import 'package:design_system_gallery/components/emoji/stream_emoji_picker_sheet.dart'
     as _design_system_gallery_components_emoji_stream_emoji_picker_sheet;
+import 'package:design_system_gallery/components/header/stream_app_bar.dart'
+    as _design_system_gallery_components_header_stream_app_bar;
 import 'package:design_system_gallery/components/header/stream_sheet_header.dart'
     as _design_system_gallery_components_header_stream_sheet_header;
 import 'package:design_system_gallery/components/message/stream_message_annotation.dart'
@@ -832,6 +834,21 @@ final directories = <_widgetbook.WidgetbookNode>[
       _widgetbook.WidgetbookFolder(
         name: 'Header',
         children: [
+          _widgetbook.WidgetbookComponent(
+            name: 'StreamAppBar',
+            useCases: [
+              _widgetbook.WidgetbookUseCase(
+                name: 'Playground',
+                builder: _design_system_gallery_components_header_stream_app_bar
+                    .buildStreamAppBarPlayground,
+              ),
+              _widgetbook.WidgetbookUseCase(
+                name: 'Showcase',
+                builder: _design_system_gallery_components_header_stream_app_bar
+                    .buildStreamAppBarShowcase,
+              ),
+            ],
+          ),
           _widgetbook.WidgetbookComponent(
             name: 'StreamSheetHeader',
             useCases: [
diff --git a/apps/design_system_gallery/lib/components/header/stream_app_bar.dart b/apps/design_system_gallery/lib/components/header/stream_app_bar.dart
new file mode 100644
index 00000000..38e6b9ce
--- /dev/null
+++ b/apps/design_system_gallery/lib/components/header/stream_app_bar.dart
@@ -0,0 +1,341 @@
+import 'package:flutter/material.dart';
+import 'package:stream_core_flutter/stream_core_flutter.dart';
+import 'package:widgetbook/widgetbook.dart';
+import 'package:widgetbook_annotation/widgetbook_annotation.dart' as widgetbook;
+
+// =============================================================================
+// Playground
+// =============================================================================
+
+@widgetbook.UseCase(
+  name: 'Playground',
+  type: StreamAppBar,
+  path: '[Components]/Header',
+)
+Widget buildStreamAppBarPlayground(BuildContext context) {
+  final title = context.knobs.stringOrNull(
+    label: 'Title',
+    initialValue: 'Details',
+    description: 'The primary header text. Clear to omit the title.',
+  );
+
+  final subtitle = context.knobs.stringOrNull(
+    label: 'Subtitle',
+    description: 'Optional second line below the title.',
+  );
+
+  final showLeading = context.knobs.boolean(
+    label: 'Show leading',
+    initialValue: true,
+    description:
+        'Renders a back-style icon button before the title. '
+        'When off, auto-implied leading only appears if the bar is '
+        'inside a poppable route (see Showcase).',
+  );
+
+  final showTrailing = context.knobs.boolean(
+    label: 'Show trailing',
+    initialValue: true,
+    description: 'Renders a primary-action button after the title.',
+  );
+
+  final padding = context.knobs.double.slider(
+    label: 'Padding',
+    initialValue: 12,
+    max: 32,
+    description: 'Uniform padding around the content row.',
+  );
+
+  final spacing = context.knobs.double.slider(
+    label: 'Spacing',
+    initialValue: 12,
+    max: 32,
+    description: 'Horizontal gap between leading, heading, and trailing.',
+  );
+
+  return Align(
+    alignment: Alignment.topCenter,
+    child: StreamAppBar(
+      style: StreamAppBarStyle(
+        padding: EdgeInsets.all(padding),
+        spacing: spacing,
+      ),
+      leading: showLeading
+          ? StreamButton.icon(
+              icon: Icon(context.streamIcons.chevronLeft),
+              style: StreamButtonStyle.secondary,
+              type: StreamButtonType.ghost,
+              onPressed: () {},
+            )
+          : null,
+      title: (title != null && title.isNotEmpty) ? Text(title) : null,
+      subtitle: (subtitle != null && subtitle.isNotEmpty) ? Text(subtitle) : null,
+      trailing: showTrailing
+          ? StreamButton.icon(
+              icon: Icon(context.streamIcons.plus),
+              onPressed: () {},
+            )
+          : null,
+    ),
+  );
+}
+
+// =============================================================================
+// Showcase
+// =============================================================================
+
+@widgetbook.UseCase(
+  name: 'Showcase',
+  type: StreamAppBar,
+  path: '[Components]/Header',
+)
+Widget buildStreamAppBarShowcase(BuildContext context) {
+  final colorScheme = context.streamColorScheme;
+  final textTheme = context.streamTextTheme;
+  final spacing = context.streamSpacing;
+
+  return DefaultTextStyle(
+    style: textTheme.bodyDefault.copyWith(color: colorScheme.textPrimary),
+    child: SingleChildScrollView(
+      padding: EdgeInsets.all(spacing.lg),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          _AppBarExample(
+            label: 'Title only',
+            bar: StreamAppBar(title: const Text('Details')),
+          ),
+          SizedBox(height: spacing.md),
+          _AppBarExample(
+            label: 'Title and subtitle',
+            bar: StreamAppBar(
+              title: const Text('Details'),
+              subtitle: const Text('Additional information'),
+            ),
+          ),
+          SizedBox(height: spacing.md),
+          _AppBarExample(
+            label: 'Leading only — trailing reserves a spacer',
+            bar: StreamAppBar(
+              leading: StreamButton.icon(
+                icon: Icon(context.streamIcons.chevronLeft),
+                style: StreamButtonStyle.secondary,
+                type: StreamButtonType.ghost,
+                onPressed: () {},
+              ),
+              title: const Text('Details'),
+            ),
+          ),
+          SizedBox(height: spacing.md),
+          _AppBarExample(
+            label: 'Trailing only — leading reserves a spacer',
+            bar: StreamAppBar(
+              title: const Text('Details'),
+              trailing: StreamButton.icon(
+                icon: Icon(context.streamIcons.plus),
+                onPressed: () {},
+              ),
+            ),
+          ),
+          SizedBox(height: spacing.md),
+          _AppBarExample(
+            label: 'Full layout with subtitle',
+            bar: StreamAppBar(
+              leading: StreamButton.icon(
+                icon: Icon(context.streamIcons.chevronLeft),
+                style: StreamButtonStyle.secondary,
+                type: StreamButtonType.ghost,
+                onPressed: () {},
+              ),
+              title: const Text('Group chat'),
+              subtitle: const Text('5 members, 3 online'),
+              trailing: StreamButton.icon(
+                icon: Icon(context.streamIcons.plus),
+                onPressed: () {},
+              ),
+            ),
+          ),
+          SizedBox(height: spacing.md),
+          _AppBarExample(
+            label: 'Long title truncates gracefully',
+            bar: StreamAppBar(
+              leading: StreamButton.icon(
+                icon: Icon(context.streamIcons.chevronLeft),
+                style: StreamButtonStyle.secondary,
+                type: StreamButtonType.ghost,
+                onPressed: () {},
+              ),
+              title: const Text(
+                'A rather long title that should ellipsize gracefully',
+                maxLines: 1,
+                overflow: TextOverflow.ellipsis,
+              ),
+              trailing: StreamButton.icon(
+                icon: Icon(context.streamIcons.plus),
+                onPressed: () {},
+              ),
+            ),
+          ),
+          SizedBox(height: spacing.md),
+          // Demonstrates the layout's centred-title behaviour: a narrow icon
+          // leading and a wide text-button trailing have very different
+          // intrinsic widths, but [StreamHeaderToolbar] reserves symmetric
+          // space around the middle so the title stays geometrically
+          // centred in the bar's full width.
+          _AppBarExample(
+            label: 'Asymmetric leading / trailing — title stays centred',
+            bar: StreamAppBar(
+              leading: StreamButton.icon(
+                icon: Icon(context.streamIcons.chevronLeft),
+                style: StreamButtonStyle.secondary,
+                type: StreamButtonType.ghost,
+                onPressed: () {},
+              ),
+              title: const Text('Group Info'),
+              trailing: StreamButton(
+                style: StreamButtonStyle.secondary,
+                type: StreamButtonType.outline,
+                size: StreamButtonSize.small,
+                onPressed: () {},
+                child: const Text('Edit'),
+              ),
+            ),
+          ),
+          SizedBox(height: spacing.md),
+          _AppBarExample(
+            label: 'Style leadingStyle/trailingStyle propagates to plain StreamButtons',
+            bar: StreamAppBar(
+              style: StreamAppBarStyle(
+                leadingStyle: StreamButtonThemeStyle.from(
+                  backgroundColor: colorScheme.backgroundSurfaceSubtle,
+                  foregroundColor: colorScheme.textPrimary,
+                ),
+                trailingStyle: StreamButtonThemeStyle.from(
+                  backgroundColor: colorScheme.accentError,
+                  foregroundColor: colorScheme.textOnAccent,
+                ),
+              ),
+              leading: StreamButton.icon(
+                icon: Icon(context.streamIcons.chevronLeft),
+                onPressed: () {},
+              ),
+              title: const Text('Discard changes?'),
+              trailing: StreamButton.icon(
+                icon: Icon(context.streamIcons.delete),
+                onPressed: () {},
+              ),
+            ),
+          ),
+          SizedBox(height: spacing.md),
+          const _AutoImplyLeadingDemo(),
+        ],
+      ),
+    ),
+  );
+}
+
+// Demonstrates the auto-implied leading button. Each launcher pushes a route
+// whose Scaffold uses a StreamAppBar with no explicit `leading`. On a regular
+// pushed page the icon adapts to the host platform — chevron on iOS / macOS,
+// arrow-left on Android / web / desktop. On a fullscreen dialog the icon is
+// always a cross. Pressing the auto-inserted button pops the route.
+class _AutoImplyLeadingDemo extends StatelessWidget {
+  const _AutoImplyLeadingDemo();
+
+  @override
+  Widget build(BuildContext context) {
+    final colorScheme = context.streamColorScheme;
+    final textTheme = context.streamTextTheme;
+    final radius = context.streamRadius;
+    final spacing = context.streamSpacing;
+
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        Text(
+          'Auto-implied leading — tap to see the pushed app bar',
+          style: textTheme.captionEmphasis.copyWith(color: colorScheme.textSecondary),
+        ),
+        SizedBox(height: spacing.xs),
+        Container(
+          decoration: BoxDecoration(
+            color: colorScheme.backgroundSurface,
+            borderRadius: BorderRadius.all(radius.lg),
+            border: Border.all(color: colorScheme.borderSubtle),
+          ),
+          padding: EdgeInsets.all(spacing.sm),
+          child: Row(
+            spacing: spacing.sm,
+            children: [
+              Expanded(
+                child: StreamButton(
+                  style: StreamButtonStyle.secondary,
+                  type: StreamButtonType.outline,
+                  onPressed: () => _push(context, fullscreenDialog: false),
+                  child: const Text('Push page'),
+                ),
+              ),
+              Expanded(
+                child: StreamButton(
+                  onPressed: () => _push(context, fullscreenDialog: true),
+                  child: const Text('Push fullscreen dialog'),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ],
+    );
+  }
+
+  void _push(BuildContext context, {required bool fullscreenDialog}) {
+    Navigator.of(context).push(
+      MaterialPageRoute<void>(
+        fullscreenDialog: fullscreenDialog,
+        builder: (_) => Scaffold(
+          appBar: StreamAppBar(
+            title: Text(fullscreenDialog ? 'Fullscreen dialog' : 'Pushed page'),
+          ),
+          body: const Center(child: Text('Pop via the auto-implied leading button.')),
+        ),
+      ),
+    );
+  }
+}
+
+class _AppBarExample extends StatelessWidget {
+  const _AppBarExample({required this.label, required this.bar});
+
+  final String label;
+  final Widget bar;
+
+  @override
+  Widget build(BuildContext context) {
+    final colorScheme = context.streamColorScheme;
+    final textTheme = context.streamTextTheme;
+    final radius = context.streamRadius;
+    final spacing = context.streamSpacing;
+
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        Text(
+          label,
+          style: textTheme.captionEmphasis.copyWith(
+            color: colorScheme.textSecondary,
+          ),
+        ),
+        SizedBox(height: spacing.xs),
+        Container(
+          clipBehavior: Clip.antiAlias,
+          decoration: BoxDecoration(
+            color: colorScheme.backgroundSurface,
+            borderRadius: BorderRadius.all(radius.lg),
+            border: Border.all(color: colorScheme.borderSubtle),
+          ),
+          child: bar,
+        ),
+      ],
+    );
+  }
+}
diff --git a/apps/design_system_gallery/lib/components/header/stream_sheet_header.dart b/apps/design_system_gallery/lib/components/header/stream_sheet_header.dart
index e963821b..8adfe616 100644
--- a/apps/design_system_gallery/lib/components/header/stream_sheet_header.dart
+++ b/apps/design_system_gallery/lib/components/header/stream_sheet_header.dart
@@ -177,6 +177,29 @@ Widget buildStreamSheetHeaderShowcase(BuildContext context) {
             ),
           ),
           SizedBox(height: spacing.md),
+          // Demonstrates the layout's centred-title behaviour: a narrow icon
+          // leading and a wide text-button trailing have very different
+          // intrinsic widths, but [StreamHeaderToolbar] reserves symmetric
+          // space around the middle so the title stays geometrically
+          // centred in the bar's full width.
+          _HeaderExample(
+            label: 'Asymmetric leading / trailing — title stays centred',
+            header: StreamSheetHeader(
+              leading: StreamButton.icon(
+                icon: Icon(context.streamIcons.xmark),
+                style: StreamButtonStyle.secondary,
+                type: StreamButtonType.outline,
+                onPressed: () {},
+              ),
+              title: const Text('Edit profile'),
+              trailing: StreamButton(
+                size: StreamButtonSize.small,
+                onPressed: () {},
+                child: const Text('Save'),
+              ),
+            ),
+          ),
+          SizedBox(height: spacing.md),
           _HeaderExample(
             label: 'Style leadingStyle/trailingStyle propagates to plain StreamButtons',
             header: StreamSheetHeader(
diff --git a/packages/stream_core_flutter/CHANGELOG.md b/packages/stream_core_flutter/CHANGELOG.md
index 315fb1e0..c4834887 100644
--- a/packages/stream_core_flutter/CHANGELOG.md
+++ b/packages/stream_core_flutter/CHANGELOG.md
@@ -7,7 +7,8 @@
 - Added `StreamFileTypeIconSize.md` and `StreamFileTypeIconSize.sm` variants.
 - Added `trailing` slot to `StreamMessageAnnotation`, with matching `trailingTextStyle`/`trailingTextColor` on `StreamMessageAnnotationStyle`.
 - Added `StreamTapTargetPadding`, a reusable primitive that grows a child's layout and hit-test area to a configurable `minSize` without changing its visual size, with a directional `alignment` that controls which direction the extra tap area extends into.
-- Added `StreamSheetHeader` component and `StreamSheetHeaderTheme` for bottom-sheet and modal headers, with auto-implied dismissal based on the enclosing route.
+- Added `StreamSheetHeader` component and `StreamSheetHeaderTheme` for bottom-sheet and modal headers, with platform-aware auto-implied dismissal based on the enclosing route.
+- Added `StreamHeaderToolbar`, a three-slot layout primitive shared by `StreamAppBar` and `StreamSheetHeader` that keeps the title geometrically centred even when leading and trailing widths differ.
 - Added `StreamSheet`, `StreamSheetDragHandle`, `StreamSheetRoute`, `StreamSheetTransition` and the `showStreamSheet` helper — Stream-styled modal bottom sheets with scroll-aware drag-to-dismiss and stacking support. `StreamSheet` can also be used standalone outside the modal route.
 - Added `StreamSheetTheme` and `StreamSheetThemeData` (`StreamTheme.sheetTheme`) for theming `StreamSheet` and modal sheets opened with `showStreamSheet`.
 - `StreamEmojiPickerSheet.show` now resolves its background color and border radius from the ambient `StreamSheetTheme` so the picker visually matches other Stream-styled sheets by default.
@@ -36,6 +37,7 @@
 - Renamed `StreamFileTypeIconSize` variants: `s48` → `xl`, `s40` → `lg`.
 - Removed `StreamMessageAnnotation.rich` and `spanTextStyle`/`spanTextColor`; use the new `trailing` slot instead.
 - Aligned `StreamButton` API with Flutter's built-in buttons: renamed `label` (`String?`) to required `child` (`Widget`), changed `icon`/`iconLeft`/`iconRight` from `IconData` to `Widget`, and renamed `onTap` to `onPressed`. `StreamButtonProps` mirrors the same renames.
+- Redesigned `StreamAppBar` with a slots-based API (`leading`/`title`/`subtitle`/`trailing`) and platform-aware auto-implied leading; replaces the previous Material `AppBar` wrapper. Adds `StreamAppBarStyle`, `StreamAppBarTheme`, and `StreamAppBarThemeData`.
 - `placeholder` on `StreamCoreMessageComposer`, `StreamMessageComposerInput`, and `StreamMessageComposerInputField` is now an optional `String?` (was `String` defaulting to `''`, and `required` on `StreamMessageComposerInputField`).
 
 ## 0.2.0
diff --git a/packages/stream_core_flutter/lib/src/components.dart b/packages/stream_core_flutter/lib/src/components.dart
index 3696cb64..6edf46a2 100644
--- a/packages/stream_core_flutter/lib/src/components.dart
+++ b/packages/stream_core_flutter/lib/src/components.dart
@@ -14,7 +14,6 @@ export 'components/badge/stream_retry_badge.dart' hide DefaultStreamRetryBadge;
 export 'components/buttons/stream_button.dart' hide DefaultStreamButton;
 export 'components/buttons/stream_emoji_button.dart' hide DefaultStreamEmojiButton;
 export 'components/buttons/stream_jump_to_unread_button.dart' hide DefaultStreamJumpToUnreadButton;
-export 'components/common/stream_app_bar.dart' hide DefaultStreamAppBar;
 export 'components/common/stream_checkbox.dart' hide DefaultStreamCheckbox;
 export 'components/common/stream_flex.dart';
 export 'components/common/stream_intrinsic_flex.dart';
@@ -38,6 +37,8 @@ export 'components/controls/stream_video_play_indicator.dart';
 export 'components/emoji/data/stream_emoji_data.dart';
 export 'components/emoji/data/stream_supported_emojis.dart';
 export 'components/emoji/stream_emoji_picker_sheet.dart';
+export 'components/header/stream_app_bar.dart' hide DefaultStreamAppBar;
+export 'components/header/stream_header_toolbar.dart';
 export 'components/header/stream_sheet_header.dart' hide DefaultStreamSheetHeader;
 export 'components/list/stream_list_tile.dart' hide DefaultStreamListTile;
 export 'components/message/stream_message_annotation.dart' hide DefaultStreamMessageAnnotation;
diff --git a/packages/stream_core_flutter/lib/src/components/common/stream_app_bar.dart b/packages/stream_core_flutter/lib/src/components/common/stream_app_bar.dart
deleted file mode 100644
index 969fc5e2..00000000
--- a/packages/stream_core_flutter/lib/src/components/common/stream_app_bar.dart
+++ /dev/null
@@ -1,220 +0,0 @@
-import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
-
-import '../../factory/stream_component_factory.dart';
-import '../../theme/semantics/stream_text_theme.dart';
-import '../../theme/stream_theme_extensions.dart';
-
-/// A styled [AppBar] with Stream defaults applied.
-///
-/// [StreamAppBar] renders a standard Material [AppBar] with
-/// defaults from the Stream design system applied.
-///
-/// {@tool snippet}
-///
-/// Display a simple app bar with a title:
-///
-/// ```dart
-/// StreamAppBar(
-///   title: Text('Messages'),
-/// )
-/// ```
-/// {@end-tool}
-///
-/// See also:
-///
-///  * [DefaultStreamAppBar], the default visual implementation.
-class StreamAppBar extends StatelessWidget implements PreferredSizeWidget {
-  /// Creates a Stream-styled app bar.
-  StreamAppBar({
-    super.key,
-    Widget? leading,
-    double? leadingWidth,
-    bool automaticallyImplyLeading = true,
-    Widget? title,
-    double? titleSpacing,
-    TextStyle? titleTextStyle,
-    List<Widget>? actions,
-    EdgeInsetsGeometry? actionsPadding,
-    bool? centerTitle,
-    PreferredSizeWidget? bottom,
-    double bottomOpacity = 1.0,
-    double elevation = 0,
-    double scrolledUnderElevation = 0,
-    Color? backgroundColor,
-    Color? surfaceTintColor,
-    ShapeBorder? shape,
-  }) : props = StreamAppBarProps(
-         leading: leading,
-         leadingWidth: leadingWidth,
-         automaticallyImplyLeading: automaticallyImplyLeading,
-         title: title,
-         titleSpacing: titleSpacing,
-         titleTextStyle: titleTextStyle,
-         actions: actions,
-         actionsPadding: actionsPadding,
-         centerTitle: centerTitle,
-         bottom: bottom,
-         bottomOpacity: bottomOpacity,
-         elevation: elevation,
-         scrolledUnderElevation: scrolledUnderElevation,
-         backgroundColor: backgroundColor,
-         surfaceTintColor: surfaceTintColor,
-         shape: shape,
-       );
-
-  /// The props controlling the appearance and behavior of this app bar.
-  final StreamAppBarProps props;
-
-  @override
-  Size get preferredSize {
-    final bottomHeight = props.bottom?.preferredSize.height ?? 0;
-    return Size.fromHeight(kToolbarHeight + bottomHeight);
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    final builder = StreamComponentFactory.of(context).appBar;
-    if (builder != null) return builder(context, props);
-    return DefaultStreamAppBar(props: props);
-  }
-}
-
-/// Properties for configuring a [StreamAppBar].
-///
-/// This class holds all the configuration options for an app bar, allowing
-/// them to be passed through the [StreamComponentFactory].
-///
-/// See also:
-///
-///  * [StreamAppBar], which uses these properties.
-///  * [DefaultStreamAppBar], the default implementation.
-class StreamAppBarProps {
-  /// Creates properties for an app bar.
-  const StreamAppBarProps({
-    this.leading,
-    this.leadingWidth,
-    this.automaticallyImplyLeading = true,
-    this.title,
-    this.titleSpacing,
-    this.titleTextStyle,
-    this.actions,
-    this.actionsPadding,
-    this.centerTitle,
-    this.bottom,
-    this.bottomOpacity = 1.0,
-    this.elevation = 0,
-    this.scrolledUnderElevation = 0,
-    this.backgroundColor,
-    this.surfaceTintColor,
-    this.shape,
-  });
-
-  /// A widget to display before the toolbar's [title].
-  final Widget? leading;
-
-  /// Defines the width of [leading] widget.
-  final double? leadingWidth;
-
-  /// Controls whether we should try to imply the leading widget if null.
-  final bool automaticallyImplyLeading;
-
-  /// The primary widget displayed in the app bar.
-  final Widget? title;
-
-  /// The spacing around [title] content on the horizontal axis.
-  final double? titleSpacing;
-
-  /// The text style for the [title].
-  ///
-  /// Defaults to [StreamTextTheme.headingSm].
-  final TextStyle? titleTextStyle;
-
-  /// {@macro flutter.material.appbar.actions}
-  final List<Widget>? actions;
-
-  /// Defines the padding for [actions].
-  final EdgeInsetsGeometry? actionsPadding;
-
-  /// Whether the title should be centered.
-  final bool? centerTitle;
-
-  /// An app bar bottom widget, displayed below the [title].
-  final PreferredSizeWidget? bottom;
-
-  /// The opacity of the [bottom] widget.
-  final double bottomOpacity;
-
-  /// The z-coordinate at which to place this app bar.
-  ///
-  /// Defaults to `0`.
-  final double elevation;
-
-  /// The elevation when content is scrolled underneath the app bar.
-  ///
-  /// Defaults to `0`.
-  final double scrolledUnderElevation;
-
-  /// The background color of the app bar.
-  final Color? backgroundColor;
-
-  /// The surface tint color of the app bar.
-  final Color? surfaceTintColor;
-
-  /// The shape of the app bar's [Material].
-  ///
-  /// Defaults to a [LinearBorder] with a bottom edge using
-  /// `borderSubtle` color from the Stream color scheme.
-  final ShapeBorder? shape;
-}
-
-/// Default implementation of [StreamAppBar].
-///
-/// Renders a Material [AppBar] with Stream design system defaults applied.
-///
-/// See also:
-///
-///  * [StreamAppBar], the public API widget.
-///  * [StreamAppBarProps], which configures this widget.
-class DefaultStreamAppBar extends StatelessWidget {
-  /// Creates a default Stream app bar.
-  const DefaultStreamAppBar({super.key, required this.props});
-
-  /// The props controlling the appearance and behavior of this app bar.
-  final StreamAppBarProps props;
-
-  @override
-  Widget build(BuildContext context) {
-    final theme = Theme.of(context);
-    final streamTextTheme = context.streamTextTheme;
-    final streamColorScheme = context.streamColorScheme;
-
-    return AppBar(
-      automaticallyImplyLeading: props.automaticallyImplyLeading,
-      toolbarTextStyle: theme.textTheme.bodyMedium,
-      titleTextStyle: props.titleTextStyle ?? streamTextTheme.headingSm,
-      systemOverlayStyle: theme.brightness == Brightness.dark ? SystemUiOverlayStyle.light : SystemUiOverlayStyle.dark,
-      elevation: props.elevation,
-      scrolledUnderElevation: props.scrolledUnderElevation,
-      backgroundColor: props.backgroundColor,
-      surfaceTintColor: props.surfaceTintColor,
-      centerTitle: props.centerTitle,
-      leading: props.leading,
-      leadingWidth: props.leadingWidth,
-      titleSpacing: props.titleSpacing,
-      actions: props.actions,
-      actionsPadding: props.actionsPadding,
-      title: props.title,
-      bottom: props.bottom,
-      bottomOpacity: props.bottomOpacity,
-      shape:
-          props.shape ??
-          LinearBorder(
-            side: BorderSide(
-              color: streamColorScheme.borderSubtle,
-            ),
-            bottom: const LinearBorderEdge(),
-          ),
-    );
-  }
-}
diff --git a/packages/stream_core_flutter/lib/src/components/header/stream_app_bar.dart b/packages/stream_core_flutter/lib/src/components/header/stream_app_bar.dart
new file mode 100644
index 00000000..bbff9834
--- /dev/null
+++ b/packages/stream_core_flutter/lib/src/components/header/stream_app_bar.dart
@@ -0,0 +1,351 @@
+import 'package:flutter/material.dart';
+
+import '../../factory/stream_component_factory.dart';
+import '../../theme/components/stream_app_bar_theme.dart';
+import '../../theme/components/stream_button_theme.dart';
+import '../../theme/primitives/stream_spacing.dart';
+import '../../theme/semantics/stream_color_scheme.dart';
+import '../../theme/semantics/stream_text_theme.dart';
+import '../../theme/stream_theme_extensions.dart';
+import '../buttons/stream_button.dart';
+import 'stream_header_toolbar.dart';
+
+/// A top-of-screen header for full-page surfaces in the Stream design system.
+///
+/// [StreamAppBar] arranges an optional centered [title] (and optional
+/// [subtitle]) between optional [leading] and [trailing] widget slots —
+/// typically a back button on the leading side and a primary action on the
+/// trailing side.
+///
+/// The heading occupies the flexible center of the row, with a 48×48 spacer
+/// reserved opposite a lone [leading] or [trailing] so the title stays
+/// visually balanced.
+///
+/// When [leading] is null and [automaticallyImplyLeading] is true (the
+/// default), a dismissal button is inserted if the enclosing route can pop:
+///
+///  * Inside a fullscreen dialog → a cross (`xmark`).
+///  * Otherwise on iOS-style platforms (iOS / macOS) → a back chevron.
+///  * Otherwise (Android / web / desktop) → an arrow-left.
+///
+/// A hairline `borderSubtle` border is drawn along the bottom edge to
+/// separate the bar from page content — it's part of the bar's identity
+/// rather than a configurable divider.
+///
+/// [StreamAppBar] implements [PreferredSizeWidget] so it can be passed
+/// directly to [Scaffold.appBar].
+///
+/// {@tool snippet}
+///
+/// Use as a [Scaffold.appBar] with a centered title — the leading back button
+/// is auto-implied:
+///
+/// ```dart
+/// Scaffold(
+///   appBar: StreamAppBar(title: const Text('Details')),
+///   body: const _DetailsBody(),
+/// )
+/// ```
+/// {@end-tool}
+///
+/// ## Theming
+///
+/// [StreamAppBar] uses [StreamAppBarThemeData] for default styling — colours,
+/// padding, spacing, title/subtitle text styles, and per-slot button style
+/// propagation. Defaults are derived from [StreamColorScheme],
+/// [StreamTextTheme], and [StreamSpacing].
+///
+/// See also:
+///
+///  * [StreamAppBarThemeData], for customizing appearance globally.
+///  * [StreamAppBarTheme], for overriding theme in a subtree.
+///  * [StreamSheetHeader], the equivalent for bottom-sheet / dialog chrome.
+///  * [DefaultStreamAppBar], the default visual implementation.
+class StreamAppBar extends StatelessWidget implements PreferredSizeWidget {
+  /// Creates a Stream app bar.
+  StreamAppBar({
+    super.key,
+    Widget? leading,
+    bool automaticallyImplyLeading = true,
+    Widget? title,
+    Widget? subtitle,
+    Widget? trailing,
+    bool primary = true,
+    StreamAppBarStyle? style,
+  }) : props = .new(
+         leading: leading,
+         automaticallyImplyLeading: automaticallyImplyLeading,
+         title: title,
+         subtitle: subtitle,
+         trailing: trailing,
+         primary: primary,
+         style: style,
+       );
+
+  /// The properties that configure this app bar.
+  final StreamAppBarProps props;
+
+  @override
+  Size get preferredSize => const Size.fromHeight(kStreamHeaderHeight);
+
+  @override
+  Widget build(BuildContext context) {
+    final builder = StreamComponentFactory.of(context).appBar;
+    if (builder != null) return builder(context, props);
+    return DefaultStreamAppBar(props: props);
+  }
+}
+
+/// Properties for configuring a [StreamAppBar].
+///
+/// This class holds all configuration options for an app bar, allowing them
+/// to be passed through the [StreamComponentFactory].
+///
+/// See also:
+///
+///  * [StreamAppBar], which uses these properties.
+///  * [DefaultStreamAppBar], the default implementation.
+class StreamAppBarProps {
+  /// Creates properties for an app bar.
+  const StreamAppBarProps({
+    this.leading,
+    this.automaticallyImplyLeading = true,
+    this.title,
+    this.subtitle,
+    this.trailing,
+    this.primary = true,
+    this.style,
+  });
+
+  /// A widget to display before the [title].
+  ///
+  /// Typically a back button. The caller is responsible for the widget's
+  /// own hit area; the app bar only reserves a 48×48 slot for symmetry.
+  ///
+  /// When null and [automaticallyImplyLeading] is true, a default dismissal
+  /// button is inserted if the enclosing route can pop — a cross on
+  /// fullscreen dialogs, a chevron on iOS-style platforms, an arrow-left
+  /// elsewhere.
+  final Widget? leading;
+
+  /// Controls whether a default dismissal button is shown when [leading] is
+  /// null.
+  ///
+  /// When true (the default), a button is inserted as the leading widget if
+  /// the enclosing route can pop. The icon depends on the surface — see
+  /// [StreamAppBar] for the full resolution table.
+  final bool automaticallyImplyLeading;
+
+  /// The primary content of the app bar.
+  ///
+  /// Typically a [Text] widget. Its text style is resolved from
+  /// [StreamAppBarStyle.titleTextStyle] (defaults to `textTheme.headingSm` on
+  /// `colorScheme.textPrimary`).
+  final Widget? title;
+
+  /// Additional content displayed below the [title].
+  ///
+  /// Typically a [Text] widget. Its text style is resolved from
+  /// [StreamAppBarStyle.subtitleTextStyle] (defaults to
+  /// `textTheme.captionDefault` on `colorScheme.textSecondary`).
+  final Widget? subtitle;
+
+  /// A widget to display after the [title].
+  ///
+  /// Typically a primary or overflow action. The caller is responsible for
+  /// the widget's own hit area; the app bar only reserves a 48×48 slot for
+  /// symmetry.
+  final Widget? trailing;
+
+  /// Whether this app bar is the topmost chrome of its surface.
+  ///
+  /// When true (the default), the app bar wraps itself in a
+  /// `SafeArea(bottom: false)` so it clears the system top inset
+  /// (status bar / notch) and horizontal insets.
+  ///
+  /// Set to false when the app bar isn't at the top of its surface (e.g.
+  /// inside a sub-section of a page that has already consumed the top
+  /// inset) so it doesn't double-pad.
+  final bool primary;
+
+  /// The visual style applied to this app bar.
+  ///
+  /// Resolution order per field: this [style] → ambient [StreamAppBarTheme]
+  /// → token-backed defaults.
+  final StreamAppBarStyle? style;
+}
+
+/// The default implementation of [StreamAppBar].
+///
+/// This widget renders the app bar with theming support from
+/// [StreamAppBarTheme]. It's used as the default factory implementation in
+/// [StreamComponentFactory].
+///
+/// The bar uses [NavigationToolbar] internally so the title is centred in
+/// the bar's full width (rather than the leftover space between leading and
+/// trailing), and only shifts when it would overlap a slot — keeping the
+/// title visually centred even when leading and trailing have different
+/// widths.
+///
+/// See also:
+///
+///  * [StreamAppBar], the public API widget.
+///  * [StreamAppBarProps], which configures this widget.
+class DefaultStreamAppBar extends StatelessWidget {
+  /// Creates a default app bar with the given [props].
+  const DefaultStreamAppBar({super.key, required this.props});
+
+  /// The properties that configure this app bar.
+  final StreamAppBarProps props;
+
+  @override
+  Widget build(BuildContext context) {
+    final icons = context.streamIcons;
+    final spacing = context.streamSpacing;
+
+    final style = context.streamAppBarTheme.style?.merge(props.style) ?? props.style;
+    final defaults = _StreamAppBarStyleDefaults(context);
+
+    final effectiveBackgroundColor = style?.backgroundColor ?? defaults.backgroundColor;
+    final effectivePadding = style?.padding ?? defaults.padding;
+    final effectiveSpacing = style?.spacing ?? defaults.spacing;
+    final effectiveTitleTextStyle = style?.titleTextStyle ?? defaults.titleTextStyle;
+    final effectiveSubtitleTextStyle = style?.subtitleTextStyle ?? defaults.subtitleTextStyle;
+    final effectiveLeadingStyle = style?.leadingStyle ?? defaults.leadingStyle;
+    final effectiveTrailingStyle = style?.trailingStyle ?? defaults.trailingStyle;
+
+    // Leading: caller-provided, or an auto-implied dismissal button when
+    // the enclosing route implies one. Fullscreen dialogs get a close
+    // cross (modal presentation); everything else gets the platform-aware
+    // back affordance.
+    var leading = props.leading;
+    if (leading == null && props.automaticallyImplyLeading) {
+      final parentRoute = ModalRoute.of(context);
+      if (parentRoute != null && parentRoute.impliesAppBarDismissal) {
+        // Platform-aware back affordance — chevron on iOS-style
+        // platforms, arrow-left elsewhere.
+        final backIcon = switch (Theme.of(context).platform) {
+          .iOS || .macOS => icons.chevronLeft,
+          _ => icons.arrowLeft,
+        };
+        final useCloseIcon = parentRoute is PageRoute && parentRoute.fullscreenDialog;
+        leading = StreamButton.icon(
+          type: .ghost,
+          style: .secondary,
+          icon: Icon(useCloseIcon ? icons.xmark : backIcon),
+          onPressed: Navigator.of(context).maybePop,
+        );
+      }
+    }
+
+    var trailing = props.trailing;
+
+    // Propagate leading/trailing button style to any StreamButton in the
+    // slot via a scoped StreamButtonTheme covering every style/type
+    // combination. Per-instance themeStyle still wins via merge.
+    if (leading != null && effectiveLeadingStyle != null) {
+      leading = StreamButtonTheme(
+        data: .all(.all(effectiveLeadingStyle)),
+        child: leading,
+      );
+    }
+
+    if (trailing != null && effectiveTrailingStyle != null) {
+      trailing = StreamButtonTheme(
+        data: .all(.all(effectiveTrailingStyle)),
+        child: trailing,
+      );
+    }
+
+    Widget? titleWidget;
+    if (props.title case final title?) {
+      titleWidget = AnimatedDefaultTextStyle(
+        style: effectiveTitleTextStyle,
+        textAlign: TextAlign.center,
+        duration: kThemeChangeDuration,
+        child: title,
+      );
+    }
+
+    Widget? subtitleWidget;
+    if (props.subtitle case final subtitle?) {
+      subtitleWidget = AnimatedDefaultTextStyle(
+        style: effectiveSubtitleTextStyle,
+        textAlign: TextAlign.center,
+        duration: kThemeChangeDuration,
+        child: subtitle,
+      );
+    }
+
+    Widget? middle;
+    if (titleWidget != null || subtitleWidget != null) {
+      middle = Column(
+        mainAxisSize: .min,
+        spacing: spacing.xxs,
+        children: [?titleWidget, ?subtitleWidget],
+      );
+    }
+
+    // The bar advertises a fixed height via [PreferredSizeWidget]; the
+    // [SizedBox] enforces it for callers that don't honour the contract
+    // (e.g. when placed directly inside a [Column] or a [Container]
+    // rather than in a [Scaffold.appBar] slot).
+    Widget bar = SizedBox(
+      height: kStreamHeaderHeight,
+      child: StreamHeaderToolbar(
+        padding: effectivePadding,
+        spacing: effectiveSpacing,
+        leading: leading,
+        middle: middle,
+        trailing: trailing,
+      ),
+    );
+
+    if (props.primary) {
+      bar = SafeArea(bottom: false, child: bar);
+    }
+
+    // The bar's bottom edge is intentionally a hairline border in the
+    // design system's `borderSubtle` colour — part of the bar's identity,
+    // not a configurable divider.
+    return DecoratedBox(
+      decoration: BoxDecoration(
+        color: effectiveBackgroundColor,
+        border: Border(
+          bottom: BorderSide(color: context.streamColorScheme.borderSubtle),
+        ),
+      ),
+      child: bar,
+    );
+  }
+}
+
+// Default style values for [StreamAppBar].
+//
+// These defaults are used when no explicit value is provided via constructor
+// parameters or [StreamAppBarStyle]. The defaults are context-aware and
+// use values from [StreamColorScheme], [StreamTextTheme], and [StreamSpacing].
+class _StreamAppBarStyleDefaults extends StreamAppBarStyle {
+  _StreamAppBarStyleDefaults(this._context);
+
+  final BuildContext _context;
+
+  late final StreamColorScheme _colorScheme = _context.streamColorScheme;
+  late final StreamTextTheme _textTheme = _context.streamTextTheme;
+  late final StreamSpacing _spacing = _context.streamSpacing;
+
+  @override
+  Color get backgroundColor => _colorScheme.backgroundElevation1;
+
+  @override
+  double get spacing => _spacing.sm;
+
+  @override
+  EdgeInsetsGeometry get padding => .all(_spacing.sm);
+
+  @override
+  TextStyle get titleTextStyle => _textTheme.headingSm.copyWith(color: _colorScheme.textPrimary);
+
+  @override
+  TextStyle get subtitleTextStyle => _textTheme.captionDefault.copyWith(color: _colorScheme.textSecondary);
+}
diff --git a/packages/stream_core_flutter/lib/src/components/header/stream_header_toolbar.dart b/packages/stream_core_flutter/lib/src/components/header/stream_header_toolbar.dart
new file mode 100644
index 00000000..30a6e5ac
--- /dev/null
+++ b/packages/stream_core_flutter/lib/src/components/header/stream_header_toolbar.dart
@@ -0,0 +1,132 @@
+import 'dart:math' as math;
+
+import 'package:flutter/material.dart';
+
+/// Default height of [StreamAppBar] and [StreamSheetHeader] per the Figma
+/// design system.
+const double kStreamHeaderHeight = 72;
+
+/// Three-slot horizontal layout shared by [StreamAppBar] and
+/// [StreamSheetHeader].
+///
+/// Lays out an optional [leading] / [trailing] flush against the bar's start
+/// and end edges (after [padding]) and an optional [middle] centred in the
+/// bar's full width — so an asymmetric leading and trailing don't shift the
+/// title off-centre. The middle is constrained to the symmetric space
+/// reserved between the side slots so it never overlaps either of them.
+///
+/// Each slot is vertically centred inside the available content height. The
+/// toolbar takes its size from the parent's tight height constraint —
+/// callers are responsible for sitting in a fixed-height slot (e.g. via
+/// [PreferredSize] or a [SizedBox] using [kStreamHeaderHeight]).
+///
+/// [padding] is the bar-edge padding around all three slots; [spacing] is
+/// the minimum gap reserved between the middle and either side slot.
+class StreamHeaderToolbar extends StatelessWidget {
+  /// Creates a header toolbar layout with the given slots.
+  const StreamHeaderToolbar({
+    super.key,
+    this.leading,
+    this.middle,
+    this.trailing,
+    this.padding = EdgeInsets.zero,
+    this.spacing = 0,
+  });
+
+  /// The widget anchored at the start edge.
+  final Widget? leading;
+
+  /// The widget centred in the bar's full inner width.
+  final Widget? middle;
+
+  /// The widget anchored at the end edge.
+  final Widget? trailing;
+
+  /// Padding applied around all three slots.
+  final EdgeInsetsGeometry padding;
+
+  /// Minimum gap reserved between the middle slot and either side slot.
+  final double spacing;
+
+  @override
+  Widget build(BuildContext context) {
+    final textDirection = Directionality.of(context);
+    return CustomMultiChildLayout(
+      delegate: _StreamHeaderToolbarLayout(
+        spacing: spacing,
+        textDirection: textDirection,
+        padding: padding.resolve(textDirection),
+      ),
+      children: [
+        if (leading != null) LayoutId(id: _Slot.leading, child: leading!),
+        if (middle != null) LayoutId(id: _Slot.middle, child: middle!),
+        if (trailing != null) LayoutId(id: _Slot.trailing, child: trailing!),
+      ],
+    );
+  }
+}
+
+enum _Slot { leading, middle, trailing }
+
+class _StreamHeaderToolbarLayout extends MultiChildLayoutDelegate {
+  _StreamHeaderToolbarLayout({
+    required this.padding,
+    required this.spacing,
+    required this.textDirection,
+  });
+
+  final EdgeInsets padding;
+  final double spacing;
+  final TextDirection textDirection;
+
+  @override
+  Size getSize(BoxConstraints constraints) => constraints.biggest;
+
+  @override
+  void performLayout(Size size) {
+    final innerLeft = padding.left;
+    final innerRight = size.width - padding.right;
+    final innerWidth = math.max<double>(0, innerRight - innerLeft);
+    final innerHeight = math.max<double>(0, size.height - padding.top - padding.bottom);
+    final isLtr = textDirection == TextDirection.ltr;
+
+    var leadingWidth = 0.0;
+    var trailingWidth = 0.0;
+
+    if (hasChild(_Slot.leading)) {
+      final slotSize = layoutChild(_Slot.leading, .loose(Size(innerWidth, innerHeight)));
+      leadingWidth = slotSize.width;
+      final dx = isLtr ? innerLeft : innerRight - leadingWidth;
+      final dy = padding.top + (innerHeight - slotSize.height) / 2;
+      positionChild(_Slot.leading, Offset(dx, dy));
+    }
+
+    if (hasChild(_Slot.trailing)) {
+      final slotSize = layoutChild(_Slot.trailing, .loose(Size(innerWidth, innerHeight)));
+      trailingWidth = slotSize.width;
+      final dx = isLtr ? innerRight - trailingWidth : innerLeft;
+      final dy = padding.top + (innerHeight - slotSize.height) / 2;
+      positionChild(_Slot.trailing, Offset(dx, dy));
+    }
+
+    if (hasChild(_Slot.middle)) {
+      // Reserve symmetric space on both sides — based on the wider of the
+      // two side slots — so the middle stays centred even when leading and
+      // trailing differ in width. Trades a slightly tighter middle for
+      // perfect centring.
+      final reservedSide = math.max(leadingWidth, trailingWidth);
+      final maxMiddleWidth = math.max<double>(0, innerWidth - 2 * reservedSide - 2 * spacing);
+      final slotSize = layoutChild(_Slot.middle, .loose(Size(maxMiddleWidth, innerHeight)));
+      final dx = (size.width - slotSize.width) / 2;
+      final dy = padding.top + (innerHeight - slotSize.height) / 2;
+      positionChild(_Slot.middle, Offset(dx, dy));
+    }
+  }
+
+  @override
+  bool shouldRelayout(covariant _StreamHeaderToolbarLayout oldDelegate) {
+    return padding != oldDelegate.padding ||
+        spacing != oldDelegate.spacing ||
+        textDirection != oldDelegate.textDirection;
+  }
+}
diff --git a/packages/stream_core_flutter/lib/src/components/header/stream_sheet_header.dart b/packages/stream_core_flutter/lib/src/components/header/stream_sheet_header.dart
index 36699088..0c0231e8 100644
--- a/packages/stream_core_flutter/lib/src/components/header/stream_sheet_header.dart
+++ b/packages/stream_core_flutter/lib/src/components/header/stream_sheet_header.dart
@@ -8,8 +8,8 @@ import '../../theme/semantics/stream_color_scheme.dart';
 import '../../theme/semantics/stream_text_theme.dart';
 import '../../theme/stream_theme_extensions.dart';
 import '../buttons/stream_button.dart';
-import '../common/stream_visibility.dart';
 import '../sheet/stream_sheet.dart';
+import 'stream_header_toolbar.dart';
 
 /// A header for bottom sheets, modals, and dialogs in the Stream design
 /// system.
@@ -31,13 +31,17 @@ import '../sheet/stream_sheet.dart';
 ///    first route), a cross (`xmark`) is shown — pressing it closes the
 ///    entire sheet.
 ///  * Inside a stacked [StreamSheetRoute] (one that covers another sheet),
-///    a back chevron is shown — pressing it pops back to the previous
-///    sheet.
-///  * Inside deeper nested routes within a [StreamSheetRoute], a back
-///    chevron is shown — pressing it pops one level inside the sheet.
+///    a platform-aware back affordance is shown — pressing it pops back to
+///    the previous sheet.
+///  * Inside deeper nested routes within a [StreamSheetRoute], a
+///    platform-aware back affordance is shown — pressing it pops one level
+///    inside the sheet.
 ///  * On any other modal surface (bottom sheets, dialogs, fullscreen
 ///    dialogs), a cross is shown.
-///  * On regular pushed pages, a back chevron is shown.
+///  * On regular pushed pages, a platform-aware back affordance is shown.
+///
+/// The platform-aware back affordance is a chevron on iOS / macOS and an
+/// arrow-left on Android / web / desktop.
 ///
 /// The drag handle shown on iOS-style bottom sheets is intentionally *not*
 /// part of this widget — the sheet itself owns that affordance, which
@@ -48,7 +52,7 @@ import '../sheet/stream_sheet.dart';
 ///
 /// Use inside a [StreamSheetRoute] with a confirm action — the leading
 /// close button is auto-implied (cross at the root of a sheet, back
-/// chevron when stacked over another sheet):
+/// affordance when stacked over another sheet):
 ///
 /// ```dart
 /// showStreamSheet<ProfileEdits>(
@@ -83,7 +87,7 @@ import '../sheet/stream_sheet.dart';
 ///  * [StreamSheetHeaderTheme], for overriding theme in a subtree.
 ///  * [StreamAppBar], the equivalent for top-level screen chrome.
 ///  * [DefaultStreamSheetHeader], the default visual implementation.
-class StreamSheetHeader extends StatelessWidget {
+class StreamSheetHeader extends StatelessWidget implements PreferredSizeWidget {
   /// Creates a Stream sheet header.
   StreamSheetHeader({
     super.key,
@@ -107,6 +111,9 @@ class StreamSheetHeader extends StatelessWidget {
   /// The properties that configure this header.
   final StreamSheetHeaderProps props;
 
+  @override
+  Size get preferredSize => const Size.fromHeight(kStreamHeaderHeight);
+
   @override
   Widget build(BuildContext context) {
     final builder = StreamComponentFactory.of(context).sheetHeader;
@@ -143,17 +150,16 @@ class StreamSheetHeaderProps {
   /// symmetry.
   ///
   /// When null and [automaticallyImplyLeading] is true, a default dismissal
-  /// button is inserted if the enclosing route can pop — a cross on modal
-  /// surfaces, a back chevron on regular pushed pages.
+  /// button is inserted if the enclosing route can pop — see
+  /// [StreamSheetHeader] for the full resolution table.
   final Widget? leading;
 
   /// Controls whether a default dismissal button is shown when [leading] is
   /// null.
   ///
   /// When true (the default), a button is inserted as the leading widget if
-  /// the enclosing route can pop. The icon is a cross on modal surfaces
-  /// (bottom sheets, dialogs, fullscreen dialogs) and a back chevron on
-  /// regular pushed pages.
+  /// the enclosing route can pop. The icon depends on the surface — see
+  /// [StreamSheetHeader] for the full resolution table.
   final bool automaticallyImplyLeading;
 
   /// The primary content of the header.
@@ -167,7 +173,7 @@ class StreamSheetHeaderProps {
   ///
   /// Typically a [Text] widget. Its text style is resolved from
   /// [StreamSheetHeaderThemeData.subtitleTextStyle] (defaults to
-  /// `textTheme.captionDefault` on `colorScheme.textTertiary`).
+  /// `textTheme.captionDefault` on `colorScheme.textSecondary`).
   final Widget? subtitle;
 
   /// A widget to display after the [title].
@@ -201,10 +207,10 @@ class StreamSheetHeaderProps {
 /// [StreamSheetHeaderTheme]. It's used as the default factory
 /// implementation in [StreamComponentFactory].
 ///
-/// When only one of [StreamSheetHeaderProps.leading] /
-/// [StreamSheetHeaderProps.trailing] is provided, the opposite side
-/// reserves a 48×48 spacer (via [StreamVisibility.hidden]) so the title
-/// stays visually centered.
+/// The title slot is centred in the header's full inner width via
+/// [StreamHeaderToolbar], which reserves symmetric space around the
+/// middle so an asymmetric leading and trailing don't shift the title
+/// off-centre.
 ///
 /// See also:
 ///
@@ -251,15 +257,22 @@ class DefaultStreamSheetHeader extends StatelessWidget {
       final parentRoute = ModalRoute.of(context);
       final sheetRoute = StreamSheetRoute.maybeOf(context);
 
+      // Platform-aware back affordance shared by every "go back" branch
+      // below — chevron on iOS-style platforms, arrow-left elsewhere.
+      final backIcon = switch (Theme.of(context).platform) {
+        .iOS || .macOS => icons.chevronLeft,
+        _ => icons.arrowLeft,
+      };
+
       IconData? icon;
       VoidCallback? onPressed;
 
       if (parentRoute is StreamSheetRoute) {
         // Header sits directly on a [StreamSheetRoute] (no nested nav
         // layer between us and the route). A stacked sheet's pop
-        // returns to the parent sheet — show a back chevron. A root
+        // returns to the parent sheet — show a back affordance. A root
         // sheet's pop dismisses it entirely — show a close cross.
-        icon = parentRoute.isStacked ? icons.chevronLeft : icons.xmark;
+        icon = parentRoute.isStacked ? backIcon : icons.xmark;
         onPressed = Navigator.of(context).maybePop;
       } else if (sheetRoute != null && parentRoute != null) {
         // Header is inside the enclosing sheet's nested navigator
@@ -267,18 +280,21 @@ class DefaultStreamSheetHeader extends StatelessWidget {
         if (parentRoute.isFirst) {
           // First nested route: tapping the icon dismisses the *whole*
           // sheet via [popSheet]. Mirror the non-nested case for the
-          // icon — chevron when the enclosing sheet is stacked (pop
-          // reveals the parent), cross when it's a root sheet.
-          icon = sheetRoute.isStacked ? icons.chevronLeft : icons.xmark;
+          // icon — back affordance when the enclosing sheet is stacked
+          // (pop reveals the parent), cross when it's a root sheet.
+          icon = sheetRoute.isStacked ? backIcon : icons.xmark;
           onPressed = () => StreamSheetRoute.popSheet(context);
         } else {
           // Deeper nested route: pop one level inside the sheet.
-          icon = icons.chevronLeft;
+          icon = backIcon;
           onPressed = Navigator.of(context).maybePop;
         }
       } else if (parentRoute != null && parentRoute.impliesAppBarDismissal) {
-        final isRegularPage = parentRoute is PageRoute && !parentRoute.fullscreenDialog;
-        icon = isRegularPage ? icons.chevronLeft : icons.xmark;
+        // Regular pushed pages get the platform-aware back affordance.
+        // Anything else that implies dismissal (popup routes, dialogs,
+        // fullscreen dialogs, custom modal routes) gets a cross.
+        final useCloseIcon = parentRoute is! PageRoute || parentRoute.fullscreenDialog;
+        icon = useCloseIcon ? icons.xmark : backIcon;
         onPressed = Navigator.of(context).maybePop;
       }
 
@@ -311,14 +327,6 @@ class DefaultStreamSheetHeader extends StatelessWidget {
       );
     }
 
-    // When only one side is present, reserve a 48×48 spacer on the opposite
-    // side so the title stays visually centered.
-    if ((leading == null) != (trailing == null)) {
-      const spacer = SizedBox.square(dimension: kMinInteractiveDimension);
-      leading ??= StreamVisibility.hidden.apply(spacer);
-      trailing ??= StreamVisibility.hidden.apply(spacer);
-    }
-
     Widget? titleWidget;
     if (props.title case final title?) {
       titleWidget = AnimatedDefaultTextStyle(
@@ -339,21 +347,27 @@ class DefaultStreamSheetHeader extends StatelessWidget {
       );
     }
 
-    Widget header = Padding(
-      padding: effectivePadding,
-      child: Row(
+    Widget? middle;
+    if (titleWidget != null || subtitleWidget != null) {
+      middle = Column(
+        mainAxisSize: .min,
+        spacing: spacing.xxs,
+        children: [?titleWidget, ?subtitleWidget],
+      );
+    }
+
+    // The header advertises a fixed height via [PreferredSizeWidget]; the
+    // [SizedBox] enforces it for callers that don't honour the contract
+    // (sheet headers usually live inside a [Column], not a slot that reads
+    // [PreferredSizeWidget.preferredSize]).
+    Widget header = SizedBox(
+      height: kStreamHeaderHeight,
+      child: StreamHeaderToolbar(
+        padding: effectivePadding,
         spacing: effectiveSpacing,
-        children: [
-          ?leading,
-          Expanded(
-            child: Column(
-              mainAxisSize: .min,
-              spacing: spacing.xxs,
-              children: [?titleWidget, ?subtitleWidget],
-            ),
-          ),
-          ?trailing,
-        ],
+        leading: leading,
+        middle: middle,
+        trailing: trailing,
       ),
     );
 
@@ -395,5 +409,5 @@ class _StreamSheetHeaderStyleDefaults extends StreamSheetHeaderStyle {
   TextStyle get titleTextStyle => _textTheme.headingSm.copyWith(color: _colorScheme.textPrimary);
 
   @override
-  TextStyle get subtitleTextStyle => _textTheme.captionDefault.copyWith(color: _colorScheme.textTertiary);
+  TextStyle get subtitleTextStyle => _textTheme.captionDefault.copyWith(color: _colorScheme.textSecondary);
 }
diff --git a/packages/stream_core_flutter/lib/src/components/sheet/stream_sheet.dart b/packages/stream_core_flutter/lib/src/components/sheet/stream_sheet.dart
index a4c01daa..a479c11f 100644
--- a/packages/stream_core_flutter/lib/src/components/sheet/stream_sheet.dart
+++ b/packages/stream_core_flutter/lib/src/components/sheet/stream_sheet.dart
@@ -615,6 +615,24 @@ class StreamSheet extends StatelessWidget {
   }
 }
 
+/// Plain value holder for the sheet's available height — the height of
+/// the slot the sheet renders into, captured during layout.
+///
+/// Drag handlers convert pixel deltas / velocities into sheet fractions
+/// by dividing by [availableHeight]. We snapshot it through a
+/// [LayoutBuilder] in [StreamSheetRoute.buildPage] (where parent
+/// constraints are stable across child rebuilds) instead of reading
+/// `context.size` at end-of-gesture, which can throw if a body
+/// `setState` marked the render object dirty mid-drag.
+///
+/// Modeled on Flutter's `DraggableScrollableSheet` extent — same idea,
+/// trimmed down to just the height field we actually need.
+class _StreamSheetExtent {
+  /// Height of the slot the sheet body fills. Set by the route's
+  /// `LayoutBuilder` and read lazily by the drag handlers.
+  double availableHeight = 0;
+}
+
 /// Modal route for a Stream-styled sheet.
 ///
 /// The sheet slides up from the bottom of the screen and rests just
@@ -756,6 +774,11 @@ class StreamSheetRoute<T> extends PageRoute<T> {
   /// Set automatically by [showStreamSheet]. Pass `null` to disable.
   final CapturedThemes? capturedThemes;
 
+  /// Tracks the sheet's available height (set by [LayoutBuilder] in
+  /// [buildPage], read by the drag handlers). Stable across body
+  /// rebuilds — see [_StreamSheetExtent].
+  final _extent = _StreamSheetExtent();
+
   /// Whether this sheet was pushed on top of another [StreamSheetRoute].
   ///
   /// Stacked sheets render a back-chevron in their auto-implied
@@ -823,13 +846,24 @@ class StreamSheetRoute<T> extends PageRoute<T> {
     Animation<double> animation,
     Animation<double> secondaryAnimation,
   ) {
-    final content = _StreamDraggableScrollableSheet(
-      enableDrag: () => enableDrag,
-      popDragController: controller!,
-      navigator: navigator!,
-      getIsCurrent: () => isCurrent,
-      getIsActive: () => isActive,
-      builder: _buildBodyWithDragHandle,
+    // Wrap the body in a LayoutBuilder so we can capture the sheet's
+    // available height during layout (constraints flow down from the
+    // route's outer SafeArea / DisplayFeatureSubScreen and are stable
+    // across body rebuilds). The drag handlers read this — never the
+    // rendered size, which can be dirty mid-gesture.
+    final content = LayoutBuilder(
+      builder: (context, constraints) {
+        _extent.availableHeight = constraints.maxHeight;
+        return _StreamDraggableScrollableSheet(
+          extent: _extent,
+          enableDrag: () => enableDrag,
+          popDragController: controller!,
+          navigator: navigator!,
+          getIsCurrent: () => isCurrent,
+          getIsActive: () => isActive,
+          builder: _buildBodyWithDragHandle,
+        );
+      },
     );
 
     Widget sheet = StreamSheet(
@@ -945,6 +979,7 @@ class StreamSheetRoute<T> extends PageRoute<T> {
     // controller.
     if (!enableDrag) {
       handle = _StreamDragGestureDetector(
+        extent: _extent,
         enabledCallback: () => true,
         onStartPopGesture: _startPopGesture,
         onDragStart: onDragStart,
@@ -953,15 +988,23 @@ class StreamSheetRoute<T> extends PageRoute<T> {
       );
     }
 
-    // The body fills the sheet; the drag handle floats over the top
-    // with a tiny offset. Stream's chrome (sheet header, etc.) is
-    // expected to leave space at the top for the handle.
+    // The drag handle floats over the top of the body with a tiny
+    // offset. Stream's chrome (sheet header, etc.) is expected to leave
+    // space at the top for the handle.
+    //
+    // The Stack uses the default [StackFit.loose] (and *not*
+    // [StackFit.expand]) so the body's intrinsic height is preserved —
+    // a body that uses [MainAxisSize.min] correctly shrink-wraps the
+    // sheet, while a body that wants to fill the screen still does so
+    // via [Expanded] / [MainAxisSize.max] from inside.
+    //
+    // The handle is placed via the Stack's [alignment] (rather than a
+    // child [Align]) so it doesn't expand to fill loose constraints —
+    // its natural 36×9 size is used and positioned at top-center of the
+    // Stack, which makes the body's size determine the Stack's size.
     return Stack(
-      fit: StackFit.expand,
-      children: <Widget>[
-        body,
-        Align(alignment: Alignment.topCenter, child: handle),
-      ],
+      alignment: Alignment.topCenter,
+      children: <Widget>[body, handle],
     );
   }
 
@@ -979,6 +1022,7 @@ class StreamSheetRoute<T> extends PageRoute<T> {
       isStacked: isStacked,
       topPadding: topPaddingFor(context),
       child: _StreamDragGestureDetector(
+        extent: _extent,
         enabledCallback: () => enableDrag,
         onStartPopGesture: _startPopGesture,
         onDragStart: onDragStart,
@@ -1059,6 +1103,7 @@ class StreamSheetDragHandle extends StatelessWidget {
 
 class _StreamDragGestureDetector extends StatefulWidget {
   const _StreamDragGestureDetector({
+    required this.extent,
     required this.enabledCallback,
     required this.onStartPopGesture,
     required this.child,
@@ -1066,6 +1111,7 @@ class _StreamDragGestureDetector extends StatefulWidget {
     this.onDragEnd,
   });
 
+  final _StreamSheetExtent extent;
   final Widget child;
   final ValueGetter<bool> enabledCallback;
   final ValueGetter<_StreamDragGestureController> onStartPopGesture;
@@ -1128,11 +1174,11 @@ class _StreamDragGestureDetectorState extends State<_StreamDragGestureDetector>
   void _handleDragUpdate(DragUpdateDetails details) {
     assert(mounted);
     assert(_dragGestureController != null);
-    final size = context.size;
-    if (size == null || size.height <= 0) return;
+    final sheetHeight = widget.extent.availableHeight;
+    if (sheetHeight <= 0) return;
     _dragGestureController?.dragUpdate(
       details.primaryDelta!,
-      sheetHeight: size.height,
+      sheetHeight: sheetHeight,
       stretchPixels: context.streamSpacing.xs,
       stretchController: _transitionScope?.stretchController,
     );
@@ -1141,8 +1187,11 @@ class _StreamDragGestureDetectorState extends State<_StreamDragGestureDetector>
   void _handleDragEnd(DragEndDetails details) {
     assert(mounted);
     assert(_dragGestureController != null);
-    final size = context.size;
-    final velocity = size != null && size.height > 0 ? details.velocity.pixelsPerSecond.dy / size.height : 0.0;
+    // Read the height the route's LayoutBuilder snapshotted during the
+    // last layout pass. Stable across body rebuilds — not affected by
+    // a mid-gesture setState that would dirty the rendered size.
+    final sheetHeight = widget.extent.availableHeight;
+    final velocity = sheetHeight > 0 ? details.velocity.pixelsPerSecond.dy / sheetHeight : 0.0;
     final isClosing =
         _dragGestureController?.dragEnd(
           velocity,
@@ -1305,6 +1354,7 @@ class _StreamDragGestureController {
 // gesture-detector path on the sheet's chrome owns its own.
 class _StreamDraggableScrollableSheet extends StatefulWidget {
   const _StreamDraggableScrollableSheet({
+    required this.extent,
     required this.enableDrag,
     required this.popDragController,
     required this.navigator,
@@ -1313,6 +1363,7 @@ class _StreamDraggableScrollableSheet extends StatefulWidget {
     required this.builder,
   });
 
+  final _StreamSheetExtent extent;
   final ValueGetter<bool> enableDrag;
   final AnimationController popDragController;
   final NavigatorState navigator;
@@ -1368,11 +1419,11 @@ class _StreamDraggableScrollableSheetState extends State<_StreamDraggableScrolla
     assert(mounted);
     final dragController = _dragGestureController;
     if (dragController == null) return;
-    final size = context.size;
-    if (size == null || size.height <= 0) return;
+    final sheetHeight = widget.extent.availableHeight;
+    if (sheetHeight <= 0) return;
     dragController.dragUpdate(
       delta,
-      sheetHeight: size.height,
+      sheetHeight: sheetHeight,
       stretchPixels: context.streamSpacing.xs,
     );
   }
@@ -1382,8 +1433,11 @@ class _StreamDraggableScrollableSheetState extends State<_StreamDraggableScrolla
     final dragController = _dragGestureController;
     if (dragController == null) return;
     _dragGestureController = null;
-    final size = context.size;
-    final sheetHeight = size != null && size.height > 0 ? size.height : 1.0;
+    // Read the height the route's LayoutBuilder snapshotted during the
+    // last layout pass. Stable across body rebuilds — not affected by
+    // a mid-gesture setState that would dirty the rendered size.
+    final cached = widget.extent.availableHeight;
+    final sheetHeight = cached > 0 ? cached : 1.0;
     // Convert scroll-position velocity (negative = finger moved down,
     // pixels/sec) to the sheet-fraction finger-down velocity that
     // [_StreamDragGestureController.dragEnd] expects.
diff --git a/packages/stream_core_flutter/lib/src/theme.dart b/packages/stream_core_flutter/lib/src/theme.dart
index 148dc3e9..fdaeffb2 100644
--- a/packages/stream_core_flutter/lib/src/theme.dart
+++ b/packages/stream_core_flutter/lib/src/theme.dart
@@ -1,5 +1,6 @@
 export 'factory/stream_component_factory.dart';
 
+export 'theme/components/stream_app_bar_theme.dart';
 export 'theme/components/stream_audio_waveform_theme.dart';
 export 'theme/components/stream_avatar_theme.dart';
 export 'theme/components/stream_badge_count_theme.dart';
diff --git a/packages/stream_core_flutter/lib/src/theme/components/stream_app_bar_theme.dart b/packages/stream_core_flutter/lib/src/theme/components/stream_app_bar_theme.dart
new file mode 100644
index 00000000..31341006
--- /dev/null
+++ b/packages/stream_core_flutter/lib/src/theme/components/stream_app_bar_theme.dart
@@ -0,0 +1,185 @@
+import 'package:flutter/widgets.dart';
+import 'package:theme_extensions_builder_annotation/theme_extensions_builder_annotation.dart';
+
+import '../stream_theme.dart';
+import 'stream_button_theme.dart';
+
+part 'stream_app_bar_theme.g.theme.dart';
+
+/// Applies an app bar theme to descendant [StreamAppBar] widgets.
+///
+/// Wrap a subtree with [StreamAppBarTheme] to override app bar styling.
+/// Access the merged theme using [BuildContext.streamAppBarTheme].
+///
+/// {@tool snippet}
+///
+/// Override app bar background for a specific subtree:
+///
+/// ```dart
+/// StreamAppBarTheme(
+///   data: StreamAppBarThemeData(
+///     style: StreamAppBarStyle(backgroundColor: Color(0xFFF6F7F9)),
+///   ),
+///   child: Scaffold(
+///     appBar: StreamAppBar(title: Text('Details')),
+///     body: ...,
+///   ),
+/// )
+/// ```
+/// {@end-tool}
+///
+/// See also:
+///
+///  * [StreamAppBarThemeData], which describes the app bar theme.
+///  * [StreamAppBarStyle], the reusable visual style embedded by the theme.
+///  * [StreamAppBar], the widget affected by this theme.
+class StreamAppBarTheme extends InheritedTheme {
+  /// Creates an app bar theme that controls descendant app bars.
+  const StreamAppBarTheme({
+    super.key,
+    required this.data,
+    required super.child,
+  });
+
+  /// The app bar theme data for descendant widgets.
+  final StreamAppBarThemeData data;
+
+  /// Returns the [StreamAppBarThemeData] merged from local and global themes.
+  ///
+  /// Local values from the nearest [StreamAppBarTheme] ancestor take
+  /// precedence over global values from [StreamTheme.of].
+  static StreamAppBarThemeData of(BuildContext context) {
+    final localTheme = context.dependOnInheritedWidgetOfExactType<StreamAppBarTheme>();
+    return StreamTheme.of(context).appBarTheme.merge(localTheme?.data);
+  }
+
+  @override
+  Widget wrap(BuildContext context, Widget child) {
+    return StreamAppBarTheme(data: data, child: child);
+  }
+
+  @override
+  bool updateShouldNotify(StreamAppBarTheme oldWidget) => data != oldWidget.data;
+}
+
+/// Theme data for customizing [StreamAppBar] widgets.
+///
+/// Wraps a [StreamAppBarStyle] so it can be served by [StreamAppBarTheme]
+/// and slotted into [StreamTheme] alongside other component theme data
+/// classes.
+///
+/// {@tool snippet}
+///
+/// Customize app bar appearance globally via [StreamTheme]:
+///
+/// ```dart
+/// StreamTheme(
+///   appBarTheme: StreamAppBarThemeData(
+///     style: StreamAppBarStyle(
+///       padding: EdgeInsets.symmetric(horizontal: 16, vertical: 12),
+///     ),
+///   ),
+/// )
+/// ```
+/// {@end-tool}
+///
+/// See also:
+///
+///  * [StreamAppBarStyle], the reusable visual style embedded here.
+///  * [StreamAppBarTheme], for overriding the theme in a widget subtree.
+///  * [StreamAppBar], the widget that uses this theme data.
+@themeGen
+@immutable
+class StreamAppBarThemeData with _$StreamAppBarThemeData {
+  /// Creates app bar theme data.
+  const StreamAppBarThemeData({this.style});
+
+  /// Visual styling for the app bar.
+  final StreamAppBarStyle? style;
+
+  /// Linearly interpolate between two [StreamAppBarThemeData] objects.
+  static StreamAppBarThemeData? lerp(
+    StreamAppBarThemeData? a,
+    StreamAppBarThemeData? b,
+    double t,
+  ) => _$StreamAppBarThemeData.lerp(a, b, t);
+}
+
+/// Visual styling properties for a [StreamAppBar].
+///
+/// Defines the appearance of the app bar — background colour, padding,
+/// inter-slot spacing, title and subtitle text styles, and per-slot button
+/// style propagation.
+///
+/// Exposed separately from [StreamAppBarThemeData] so other theme data classes
+/// can embed an app-bar style via a typed field.
+///
+/// {@tool snippet}
+///
+/// Compose a style and hand it to an app bar theme:
+///
+/// ```dart
+/// StreamAppBarStyle(
+///   backgroundColor: Color(0xFFFFFFFF),
+///   padding: EdgeInsets.symmetric(horizontal: 16, vertical: 12),
+///   spacing: 8,
+/// )
+/// ```
+/// {@end-tool}
+///
+/// See also:
+///
+///  * [StreamAppBarThemeData], which wraps this style for theming.
+///  * [StreamAppBar], which uses this styling.
+@themeGen
+@immutable
+class StreamAppBarStyle with _$StreamAppBarStyle {
+  /// Creates an app bar style with optional property overrides.
+  const StreamAppBarStyle({
+    this.backgroundColor,
+    this.padding,
+    this.spacing,
+    this.titleTextStyle,
+    this.subtitleTextStyle,
+    this.leadingStyle,
+    this.trailingStyle,
+  });
+
+  /// The background colour of the app bar.
+  final Color? backgroundColor;
+
+  /// The padding around the header's content row.
+  final EdgeInsetsGeometry? padding;
+
+  /// The horizontal space between the leading, heading, and trailing slots.
+  final double? spacing;
+
+  /// The text style for [StreamAppBar.title].
+  final TextStyle? titleTextStyle;
+
+  /// The text style for [StreamAppBar.subtitle].
+  final TextStyle? subtitleTextStyle;
+
+  /// The button style for any [StreamButton] rendered in
+  /// [StreamAppBar.leading].
+  ///
+  /// Applied via a scoped [StreamButtonTheme] so any [StreamButton] dropped
+  /// into the slot picks it up regardless of the button's configured `style`
+  /// or `type`. Per-instance `themeStyle` overrides still win via merge.
+  final StreamButtonThemeStyle? leadingStyle;
+
+  /// The button style for any [StreamButton] rendered in
+  /// [StreamAppBar.trailing].
+  ///
+  /// Applied via a scoped [StreamButtonTheme] so any [StreamButton] dropped
+  /// into the slot picks it up regardless of the button's configured `style`
+  /// or `type`. Per-instance `themeStyle` overrides still win via merge.
+  final StreamButtonThemeStyle? trailingStyle;
+
+  /// Linearly interpolate between two [StreamAppBarStyle] objects.
+  static StreamAppBarStyle? lerp(
+    StreamAppBarStyle? a,
+    StreamAppBarStyle? b,
+    double t,
+  ) => _$StreamAppBarStyle.lerp(a, b, t);
+}
diff --git a/packages/stream_core_flutter/lib/src/theme/components/stream_app_bar_theme.g.theme.dart b/packages/stream_core_flutter/lib/src/theme/components/stream_app_bar_theme.g.theme.dart
new file mode 100644
index 00000000..5f0e722e
--- /dev/null
+++ b/packages/stream_core_flutter/lib/src/theme/components/stream_app_bar_theme.g.theme.dart
@@ -0,0 +1,212 @@
+// dart format width=80
+// coverage:ignore-file
+// GENERATED CODE - DO NOT MODIFY BY HAND
+// ignore_for_file: type=lint, unused_element
+
+part of 'stream_app_bar_theme.dart';
+
+// **************************************************************************
+// ThemeGenGenerator
+// **************************************************************************
+
+mixin _$StreamAppBarThemeData {
+  bool get canMerge => true;
+
+  static StreamAppBarThemeData? lerp(
+    StreamAppBarThemeData? a,
+    StreamAppBarThemeData? b,
+    double t,
+  ) {
+    if (identical(a, b)) {
+      return a;
+    }
+
+    if (a == null) {
+      return t == 1.0 ? b : null;
+    }
+
+    if (b == null) {
+      return t == 0.0 ? a : null;
+    }
+
+    return StreamAppBarThemeData(
+      style: StreamAppBarStyle.lerp(a.style, b.style, t),
+    );
+  }
+
+  StreamAppBarThemeData copyWith({StreamAppBarStyle? style}) {
+    final _this = (this as StreamAppBarThemeData);
+
+    return StreamAppBarThemeData(style: style ?? _this.style);
+  }
+
+  StreamAppBarThemeData merge(StreamAppBarThemeData? other) {
+    final _this = (this as StreamAppBarThemeData);
+
+    if (other == null || identical(_this, other)) {
+      return _this;
+    }
+
+    if (!other.canMerge) {
+      return other;
+    }
+
+    return copyWith(style: _this.style?.merge(other.style) ?? other.style);
+  }
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) {
+      return true;
+    }
+
+    if (other.runtimeType != runtimeType) {
+      return false;
+    }
+
+    final _this = (this as StreamAppBarThemeData);
+    final _other = (other as StreamAppBarThemeData);
+
+    return _other.style == _this.style;
+  }
+
+  @override
+  int get hashCode {
+    final _this = (this as StreamAppBarThemeData);
+
+    return Object.hash(runtimeType, _this.style);
+  }
+}
+
+mixin _$StreamAppBarStyle {
+  bool get canMerge => true;
+
+  static StreamAppBarStyle? lerp(
+    StreamAppBarStyle? a,
+    StreamAppBarStyle? b,
+    double t,
+  ) {
+    if (identical(a, b)) {
+      return a;
+    }
+
+    if (a == null) {
+      return t == 1.0 ? b : null;
+    }
+
+    if (b == null) {
+      return t == 0.0 ? a : null;
+    }
+
+    return StreamAppBarStyle(
+      backgroundColor: Color.lerp(a.backgroundColor, b.backgroundColor, t),
+      padding: EdgeInsetsGeometry.lerp(a.padding, b.padding, t),
+      spacing: lerpDouble$(a.spacing, b.spacing, t),
+      titleTextStyle: TextStyle.lerp(a.titleTextStyle, b.titleTextStyle, t),
+      subtitleTextStyle: TextStyle.lerp(
+        a.subtitleTextStyle,
+        b.subtitleTextStyle,
+        t,
+      ),
+      leadingStyle: StreamButtonThemeStyle.lerp(
+        a.leadingStyle,
+        b.leadingStyle,
+        t,
+      ),
+      trailingStyle: StreamButtonThemeStyle.lerp(
+        a.trailingStyle,
+        b.trailingStyle,
+        t,
+      ),
+    );
+  }
+
+  StreamAppBarStyle copyWith({
+    Color? backgroundColor,
+    EdgeInsetsGeometry? padding,
+    double? spacing,
+    TextStyle? titleTextStyle,
+    TextStyle? subtitleTextStyle,
+    StreamButtonThemeStyle? leadingStyle,
+    StreamButtonThemeStyle? trailingStyle,
+  }) {
+    final _this = (this as StreamAppBarStyle);
+
+    return StreamAppBarStyle(
+      backgroundColor: backgroundColor ?? _this.backgroundColor,
+      padding: padding ?? _this.padding,
+      spacing: spacing ?? _this.spacing,
+      titleTextStyle: titleTextStyle ?? _this.titleTextStyle,
+      subtitleTextStyle: subtitleTextStyle ?? _this.subtitleTextStyle,
+      leadingStyle: leadingStyle ?? _this.leadingStyle,
+      trailingStyle: trailingStyle ?? _this.trailingStyle,
+    );
+  }
+
+  StreamAppBarStyle merge(StreamAppBarStyle? other) {
+    final _this = (this as StreamAppBarStyle);
+
+    if (other == null || identical(_this, other)) {
+      return _this;
+    }
+
+    if (!other.canMerge) {
+      return other;
+    }
+
+    return copyWith(
+      backgroundColor: other.backgroundColor,
+      padding: other.padding,
+      spacing: other.spacing,
+      titleTextStyle:
+          _this.titleTextStyle?.merge(other.titleTextStyle) ??
+          other.titleTextStyle,
+      subtitleTextStyle:
+          _this.subtitleTextStyle?.merge(other.subtitleTextStyle) ??
+          other.subtitleTextStyle,
+      leadingStyle:
+          _this.leadingStyle?.merge(other.leadingStyle) ?? other.leadingStyle,
+      trailingStyle:
+          _this.trailingStyle?.merge(other.trailingStyle) ??
+          other.trailingStyle,
+    );
+  }
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) {
+      return true;
+    }
+
+    if (other.runtimeType != runtimeType) {
+      return false;
+    }
+
+    final _this = (this as StreamAppBarStyle);
+    final _other = (other as StreamAppBarStyle);
+
+    return _other.backgroundColor == _this.backgroundColor &&
+        _other.padding == _this.padding &&
+        _other.spacing == _this.spacing &&
+        _other.titleTextStyle == _this.titleTextStyle &&
+        _other.subtitleTextStyle == _this.subtitleTextStyle &&
+        _other.leadingStyle == _this.leadingStyle &&
+        _other.trailingStyle == _this.trailingStyle;
+  }
+
+  @override
+  int get hashCode {
+    final _this = (this as StreamAppBarStyle);
+
+    return Object.hash(
+      runtimeType,
+      _this.backgroundColor,
+      _this.padding,
+      _this.spacing,
+      _this.titleTextStyle,
+      _this.subtitleTextStyle,
+      _this.leadingStyle,
+      _this.trailingStyle,
+    );
+  }
+}
diff --git a/packages/stream_core_flutter/lib/src/theme/stream_theme.dart b/packages/stream_core_flutter/lib/src/theme/stream_theme.dart
index 0b370595..ffd5eeb9 100644
--- a/packages/stream_core_flutter/lib/src/theme/stream_theme.dart
+++ b/packages/stream_core_flutter/lib/src/theme/stream_theme.dart
@@ -4,6 +4,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:theme_extensions_builder_annotation/theme_extensions_builder_annotation.dart';
 
+import 'components/stream_app_bar_theme.dart';
 import 'components/stream_audio_waveform_theme.dart';
 import 'components/stream_avatar_theme.dart';
 import 'components/stream_badge_count_theme.dart';
@@ -104,6 +105,7 @@ class StreamTheme extends ThemeExtension<StreamTheme> with _$StreamTheme {
     StreamTextTheme? textTheme,
     StreamBoxShadow? boxShadow,
     // Components themes
+    StreamAppBarThemeData? appBarTheme,
     StreamAudioWaveformThemeData? audioWaveformTheme,
     StreamAvatarThemeData? avatarTheme,
     StreamBadgeCountThemeData? badgeCountTheme,
@@ -146,6 +148,7 @@ class StreamTheme extends ThemeExtension<StreamTheme> with _$StreamTheme {
     boxShadow ??= isDark ? StreamBoxShadow.dark() : StreamBoxShadow.light();
 
     // Components
+    appBarTheme ??= const StreamAppBarThemeData();
     audioWaveformTheme ??= const StreamAudioWaveformThemeData();
     avatarTheme ??= const StreamAvatarThemeData();
     badgeCountTheme ??= const StreamBadgeCountThemeData();
@@ -182,6 +185,7 @@ class StreamTheme extends ThemeExtension<StreamTheme> with _$StreamTheme {
       colorScheme: colorScheme,
       textTheme: textTheme,
       boxShadow: boxShadow,
+      appBarTheme: appBarTheme,
       audioWaveformTheme: audioWaveformTheme,
       avatarTheme: avatarTheme,
       badgeCountTheme: badgeCountTheme,
@@ -232,6 +236,7 @@ class StreamTheme extends ThemeExtension<StreamTheme> with _$StreamTheme {
     required this.colorScheme,
     required this.textTheme,
     required this.boxShadow,
+    required this.appBarTheme,
     required this.audioWaveformTheme,
     required this.avatarTheme,
     required this.badgeCountTheme,
@@ -318,6 +323,9 @@ class StreamTheme extends ThemeExtension<StreamTheme> with _$StreamTheme {
   /// The box shadow (elevation) values for this theme.
   final StreamBoxShadow boxShadow;
 
+  /// The app bar theme for this theme.
+  final StreamAppBarThemeData appBarTheme;
+
   /// The audio waveform theme for this theme.
   final StreamAudioWaveformThemeData audioWaveformTheme;
 
@@ -426,6 +434,7 @@ class StreamTheme extends ThemeExtension<StreamTheme> with _$StreamTheme {
       colorScheme: colorScheme,
       textTheme: newTextTheme,
       boxShadow: boxShadow,
+      appBarTheme: appBarTheme,
       audioWaveformTheme: audioWaveformTheme,
       avatarTheme: avatarTheme,
       badgeCountTheme: badgeCountTheme,
diff --git a/packages/stream_core_flutter/lib/src/theme/stream_theme.g.theme.dart b/packages/stream_core_flutter/lib/src/theme/stream_theme.g.theme.dart
index a3f644a3..fdc48da8 100644
--- a/packages/stream_core_flutter/lib/src/theme/stream_theme.g.theme.dart
+++ b/packages/stream_core_flutter/lib/src/theme/stream_theme.g.theme.dart
@@ -20,6 +20,7 @@ mixin _$StreamTheme on ThemeExtension<StreamTheme> {
     StreamColorScheme? colorScheme,
     StreamTextTheme? textTheme,
     StreamBoxShadow? boxShadow,
+    StreamAppBarThemeData? appBarTheme,
     StreamAudioWaveformThemeData? audioWaveformTheme,
     StreamAvatarThemeData? avatarTheme,
     StreamBadgeCountThemeData? badgeCountTheme,
@@ -58,6 +59,7 @@ mixin _$StreamTheme on ThemeExtension<StreamTheme> {
       colorScheme: colorScheme ?? _this.colorScheme,
       textTheme: textTheme ?? _this.textTheme,
       boxShadow: boxShadow ?? _this.boxShadow,
+      appBarTheme: appBarTheme ?? _this.appBarTheme,
       audioWaveformTheme: audioWaveformTheme ?? _this.audioWaveformTheme,
       avatarTheme: avatarTheme ?? _this.avatarTheme,
       badgeCountTheme: badgeCountTheme ?? _this.badgeCountTheme,
@@ -112,6 +114,11 @@ mixin _$StreamTheme on ThemeExtension<StreamTheme> {
           (_this.colorScheme.lerp(other.colorScheme, t) as StreamColorScheme),
       textTheme: (_this.textTheme.lerp(other.textTheme, t) as StreamTextTheme),
       boxShadow: StreamBoxShadow.lerp(_this.boxShadow, other.boxShadow, t)!,
+      appBarTheme: StreamAppBarThemeData.lerp(
+        _this.appBarTheme,
+        other.appBarTheme,
+        t,
+      )!,
       audioWaveformTheme: StreamAudioWaveformThemeData.lerp(
         _this.audioWaveformTheme,
         other.audioWaveformTheme,
@@ -262,6 +269,7 @@ mixin _$StreamTheme on ThemeExtension<StreamTheme> {
         _other.colorScheme == _this.colorScheme &&
         _other.textTheme == _this.textTheme &&
         _other.boxShadow == _this.boxShadow &&
+        _other.appBarTheme == _this.appBarTheme &&
         _other.audioWaveformTheme == _this.audioWaveformTheme &&
         _other.avatarTheme == _this.avatarTheme &&
         _other.badgeCountTheme == _this.badgeCountTheme &&
@@ -304,6 +312,7 @@ mixin _$StreamTheme on ThemeExtension<StreamTheme> {
       _this.colorScheme,
       _this.textTheme,
       _this.boxShadow,
+      _this.appBarTheme,
       _this.audioWaveformTheme,
       _this.avatarTheme,
       _this.badgeCountTheme,
diff --git a/packages/stream_core_flutter/lib/src/theme/stream_theme_extensions.dart b/packages/stream_core_flutter/lib/src/theme/stream_theme_extensions.dart
index 4e357e4f..97c69198 100644
--- a/packages/stream_core_flutter/lib/src/theme/stream_theme_extensions.dart
+++ b/packages/stream_core_flutter/lib/src/theme/stream_theme_extensions.dart
@@ -1,5 +1,6 @@
 import 'package:flutter/widgets.dart';
 
+import 'components/stream_app_bar_theme.dart';
 import 'components/stream_audio_waveform_theme.dart';
 import 'components/stream_avatar_theme.dart';
 import 'components/stream_badge_count_theme.dart';
@@ -80,6 +81,9 @@ extension StreamThemeExtension on BuildContext {
   /// Returns the [StreamBoxShadow] from the current theme.
   StreamBoxShadow get streamBoxShadow => streamTheme.boxShadow;
 
+  /// Returns the [StreamAppBarThemeData] from the nearest ancestor.
+  StreamAppBarThemeData get streamAppBarTheme => StreamAppBarTheme.of(this);
+
   /// Returns the [StreamAudioWaveformThemeData] from the nearest ancestor.
   StreamAudioWaveformThemeData get streamAudioWaveformTheme => StreamAudioWaveformTheme.of(this);
 
diff --git a/packages/stream_core_flutter/test/components/header/goldens/ci/stream_sheet_header_dark_matrix.png b/packages/stream_core_flutter/test/components/header/goldens/ci/stream_sheet_header_dark_matrix.png
deleted file mode 100644
index 3aaf5992..00000000
Binary files a/packages/stream_core_flutter/test/components/header/goldens/ci/stream_sheet_header_dark_matrix.png and /dev/null differ
diff --git a/packages/stream_core_flutter/test/components/header/goldens/ci/stream_sheet_header_light_matrix.png b/packages/stream_core_flutter/test/components/header/goldens/ci/stream_sheet_header_light_matrix.png
deleted file mode 100644
index b5ad58c2..00000000
Binary files a/packages/stream_core_flutter/test/components/header/goldens/ci/stream_sheet_header_light_matrix.png and /dev/null differ
diff --git a/packages/stream_core_flutter/test/components/header/stream_app_bar_test.dart b/packages/stream_core_flutter/test/components/header/stream_app_bar_test.dart
new file mode 100644
index 00000000..bc4fb3e1
--- /dev/null
+++ b/packages/stream_core_flutter/test/components/header/stream_app_bar_test.dart
@@ -0,0 +1,192 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:stream_core_flutter/stream_core_flutter.dart';
+
+Widget _withStreamTheme(Widget child) {
+  return MaterialApp(
+    theme: ThemeData(extensions: [StreamTheme()]),
+    home: child,
+  );
+}
+
+void main() {
+  group('StreamAppBar auto-implied leading', () {
+    testWidgets('does nothing when route cannot pop', (tester) async {
+      await tester.pumpWidget(
+        _withStreamTheme(Scaffold(appBar: StreamAppBar(title: const Text('Title')))),
+      );
+
+      // Root route of MaterialApp — canPop() is false, so no back button.
+      expect(find.byType(StreamButton), findsNothing);
+    });
+
+    testWidgets('inserts arrow-left on a regular pushed route on Android', (tester) async {
+      debugDefaultTargetPlatformOverride = TargetPlatform.android;
+      try {
+        await tester.pumpWidget(_withStreamTheme(const _LauncherScreen()));
+        await tester.tap(find.text('Open'));
+        await tester.pumpAndSettle();
+
+        expect(find.byType(StreamButton), findsOneWidget);
+        expect(find.byIcon(StreamIconData.arrowLeft), findsOneWidget);
+        expect(find.byIcon(StreamIconData.chevronLeft), findsNothing);
+        expect(find.byIcon(StreamIconData.xmark), findsNothing);
+      } finally {
+        debugDefaultTargetPlatformOverride = null;
+      }
+    });
+
+    testWidgets('inserts back chevron on a regular pushed route on iOS', (tester) async {
+      debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+      try {
+        await tester.pumpWidget(_withStreamTheme(const _LauncherScreen()));
+        await tester.tap(find.text('Open'));
+        await tester.pumpAndSettle();
+
+        expect(find.byType(StreamButton), findsOneWidget);
+        expect(find.byIcon(StreamIconData.chevronLeft), findsOneWidget);
+        expect(find.byIcon(StreamIconData.arrowLeft), findsNothing);
+        expect(find.byIcon(StreamIconData.xmark), findsNothing);
+      } finally {
+        debugDefaultTargetPlatformOverride = null;
+      }
+    });
+
+    testWidgets('inserts cross icon on a fullscreen dialog', (tester) async {
+      await tester.pumpWidget(_withStreamTheme(const _LauncherScreen(fullscreenDialog: true)));
+      await tester.tap(find.text('Open'));
+      await tester.pumpAndSettle();
+
+      expect(find.byType(StreamButton), findsOneWidget);
+      expect(find.byIcon(StreamIconData.xmark), findsOneWidget);
+      expect(find.byIcon(StreamIconData.chevronLeft), findsNothing);
+    });
+
+    testWidgets('caller-provided leading suppresses the default', (tester) async {
+      await tester.pumpWidget(_withStreamTheme(const _LauncherScreen(customLeading: true)));
+      await tester.tap(find.text('Open'));
+      await tester.pumpAndSettle();
+
+      // One, not two — the caller's widget replaces the default.
+      expect(find.byKey(const ValueKey('custom-leading')), findsOneWidget);
+      expect(find.byType(StreamButton), findsNothing);
+    });
+
+    testWidgets('automaticallyImplyLeading: false suppresses the default', (tester) async {
+      await tester.pumpWidget(_withStreamTheme(const _LauncherScreen(implyLeading: false)));
+      await tester.tap(find.text('Open'));
+      await tester.pumpAndSettle();
+
+      expect(find.byType(StreamButton), findsNothing);
+    });
+  });
+
+  group('StreamAppBar style precedence', () {
+    testWidgets(
+      'props.style > theme.style > token defaults (three-level merge)',
+      (tester) async {
+        const propsPadding = EdgeInsets.all(7);
+        const themeTitleStyle = TextStyle(fontSize: 18, color: Color(0xFF112233));
+
+        await tester.pumpWidget(
+          MaterialApp(
+            theme: ThemeData(extensions: [StreamTheme()]),
+            home: StreamAppBarTheme(
+              data: const StreamAppBarThemeData(
+                style: StreamAppBarStyle(titleTextStyle: themeTitleStyle),
+              ),
+              child: Scaffold(
+                appBar: StreamAppBar(
+                  automaticallyImplyLeading: false,
+                  title: const Text('Title'),
+                  subtitle: const Text('Subtitle'),
+                  style: const StreamAppBarStyle(padding: propsPadding),
+                ),
+              ),
+            ),
+          ),
+        );
+
+        // Props win for padding (the bar passes its resolved padding through
+        // to the [StreamHeaderToolbar]'s `padding` property).
+        final toolbar = tester.widget<StreamHeaderToolbar>(
+          find.descendant(
+            of: find.byType(StreamAppBar),
+            matching: find.byType(StreamHeaderToolbar),
+          ),
+        );
+        expect(toolbar.padding, equals(propsPadding));
+
+        // Theme wins for titleTextStyle (props didn't set it).
+        final titleStyle = tester
+            .widget<DefaultTextStyle>(
+              find
+                  .ancestor(
+                    of: find.text('Title'),
+                    matching: find.byType(DefaultTextStyle),
+                  )
+                  .first,
+            )
+            .style;
+        expect(titleStyle.fontSize, equals(themeTitleStyle.fontSize));
+        expect(titleStyle.color, equals(themeTitleStyle.color));
+
+        // Subtitle falls through to defaults (neither props nor theme set it).
+        final subtitleStyle = tester
+            .widget<DefaultTextStyle>(
+              find
+                  .ancestor(
+                    of: find.text('Subtitle'),
+                    matching: find.byType(DefaultTextStyle),
+                  )
+                  .first,
+            )
+            .style;
+        expect(subtitleStyle.fontSize, isNotNull);
+        expect(subtitleStyle.fontSize, greaterThan(0));
+      },
+    );
+  });
+}
+
+class _LauncherScreen extends StatelessWidget {
+  const _LauncherScreen({
+    this.customLeading = false,
+    this.implyLeading = true,
+    this.fullscreenDialog = false,
+  });
+
+  final bool customLeading;
+  final bool implyLeading;
+  final bool fullscreenDialog;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Center(
+        child: Builder(
+          builder: (context) => TextButton(
+            onPressed: () {
+              Navigator.of(context).push(
+                MaterialPageRoute<void>(
+                  fullscreenDialog: fullscreenDialog,
+                  builder: (_) => Scaffold(
+                    appBar: StreamAppBar(
+                      automaticallyImplyLeading: implyLeading,
+                      leading: customLeading
+                          ? const SizedBox(key: ValueKey('custom-leading'), width: 40, height: 40)
+                          : null,
+                      title: const Text('Pushed'),
+                    ),
+                  ),
+                ),
+              );
+            },
+            child: const Text('Open'),
+          ),
+        ),
+      ),
+    );
+  }
+}
diff --git a/packages/stream_core_flutter/test/components/header/stream_sheet_header_golden_test.dart b/packages/stream_core_flutter/test/components/header/stream_sheet_header_golden_test.dart
deleted file mode 100644
index b341582a..00000000
--- a/packages/stream_core_flutter/test/components/header/stream_sheet_header_golden_test.dart
+++ /dev/null
@@ -1,143 +0,0 @@
-import 'package:alchemist/alchemist.dart';
-import 'package:flutter/material.dart';
-import 'package:flutter_test/flutter_test.dart';
-import 'package:stream_core_flutter/stream_core_flutter.dart';
-
-void main() {
-  group('StreamSheetHeader Golden Tests', () {
-    goldenTest(
-      'renders light theme slot matrix',
-      fileName: 'stream_sheet_header_light_matrix',
-      builder: () => GoldenTestGroup(
-        scenarioConstraints: const BoxConstraints(maxWidth: 360),
-        children: [
-          for (final scenario in _scenarios)
-            GoldenTestScenario(
-              name: scenario.name,
-              child: _buildInTheme(scenario.build()),
-            ),
-        ],
-      ),
-    );
-
-    goldenTest(
-      'renders dark theme slot matrix',
-      fileName: 'stream_sheet_header_dark_matrix',
-      builder: () => GoldenTestGroup(
-        scenarioConstraints: const BoxConstraints(maxWidth: 360),
-        children: [
-          for (final scenario in _scenarios)
-            GoldenTestScenario(
-              name: scenario.name,
-              child: _buildInTheme(
-                scenario.build(),
-                brightness: Brightness.dark,
-              ),
-            ),
-        ],
-      ),
-    );
-  });
-}
-
-typedef _Scenario = ({String name, Widget Function() build});
-
-final List<_Scenario> _scenarios = [
-  (
-    name: 'title_only',
-    build: () => StreamSheetHeader(title: const Text('Details')),
-  ),
-  (
-    name: 'title_and_subtitle',
-    build: () => StreamSheetHeader(
-      title: const Text('Details'),
-      subtitle: const Text('Additional information'),
-    ),
-  ),
-  (
-    name: 'title_with_leading',
-    build: () => StreamSheetHeader(
-      leading: _placeholder(),
-      title: const Text('Details'),
-    ),
-  ),
-  (
-    name: 'title_with_trailing',
-    build: () => StreamSheetHeader(
-      title: const Text('Details'),
-      trailing: _placeholder(color: const Color(0xFF005FFF)),
-    ),
-  ),
-  (
-    name: 'title_with_both_sides',
-    build: () => StreamSheetHeader(
-      leading: _placeholder(),
-      title: const Text('Details'),
-      trailing: _placeholder(color: const Color(0xFF005FFF)),
-    ),
-  ),
-  (
-    name: 'full_with_subtitle',
-    build: () => StreamSheetHeader(
-      leading: _placeholder(),
-      title: const Text('Edit profile'),
-      subtitle: const Text('Tap to change your avatar'),
-      trailing: _placeholder(color: const Color(0xFF005FFF)),
-    ),
-  ),
-  (
-    name: 'long_title_with_both_sides',
-    build: () => StreamSheetHeader(
-      leading: _placeholder(),
-      title: const Text(
-        'A rather long title that should ellipsize gracefully',
-        maxLines: 1,
-        overflow: TextOverflow.ellipsis,
-      ),
-      trailing: _placeholder(color: const Color(0xFF005FFF)),
-    ),
-  ),
-  (
-    name: 'no_title_only_actions',
-    build: () => StreamSheetHeader(
-      leading: _placeholder(),
-      trailing: _placeholder(color: const Color(0xFF005FFF)),
-    ),
-  ),
-  (
-    name: 'subtitle_only',
-    build: () => StreamSheetHeader(
-      subtitle: const Text('Subtitle without a title'),
-    ),
-  ),
-];
-
-Widget _placeholder({Color color = const Color(0xFFD5DBE1)}) {
-  return Container(
-    width: 40,
-    height: 40,
-    decoration: BoxDecoration(
-      color: color,
-      shape: BoxShape.circle,
-    ),
-  );
-}
-
-Widget _buildInTheme(
-  Widget child, {
-  Brightness brightness = Brightness.light,
-}) {
-  final streamTheme = StreamTheme(brightness: brightness);
-  return Theme(
-    data: ThemeData(
-      brightness: brightness,
-      extensions: [streamTheme],
-    ),
-    child: Builder(
-      builder: (context) => Material(
-        color: StreamTheme.of(context).colorScheme.backgroundApp,
-        child: child,
-      ),
-    ),
-  );
-}
diff --git a/packages/stream_core_flutter/test/components/header/stream_sheet_header_test.dart b/packages/stream_core_flutter/test/components/header/stream_sheet_header_test.dart
index a5b297f1..a9ba97ea 100644
--- a/packages/stream_core_flutter/test/components/header/stream_sheet_header_test.dart
+++ b/packages/stream_core_flutter/test/components/header/stream_sheet_header_test.dart
@@ -1,5 +1,6 @@
 import 'dart:async';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:stream_core_flutter/stream_core_flutter.dart';
@@ -11,6 +12,21 @@ Widget _withStreamTheme(Widget child) {
   );
 }
 
+// Wraps [body] in a try/finally that pins the platform for the duration
+// of the test and clears the override before flutter_test's end-of-test
+// `debugAssertAllFoundationVarsUnset` assertion runs.
+Future<void> _onPlatform(
+  TargetPlatform platform,
+  Future<void> Function() body,
+) async {
+  debugDefaultTargetPlatformOverride = platform;
+  try {
+    await body();
+  } finally {
+    debugDefaultTargetPlatformOverride = null;
+  }
+}
+
 void main() {
   group('StreamSheetHeader auto-implied leading', () {
     testWidgets('does nothing when route cannot pop', (tester) async {
@@ -24,14 +40,36 @@ void main() {
       expect(find.byType(StreamButton), findsNothing);
     });
 
-    testWidgets('inserts back chevron on a regular pushed route', (tester) async {
-      await tester.pumpWidget(_withStreamTheme(const _LauncherScreen()));
-      await tester.tap(find.text('Open'));
-      await tester.pumpAndSettle();
+    testWidgets('inserts arrow-left on a regular pushed route on Android', (tester) async {
+      debugDefaultTargetPlatformOverride = TargetPlatform.android;
+      try {
+        await tester.pumpWidget(_withStreamTheme(const _LauncherScreen()));
+        await tester.tap(find.text('Open'));
+        await tester.pumpAndSettle();
 
-      expect(find.byType(StreamButton), findsOneWidget);
-      expect(find.byIcon(StreamIconData.chevronLeft), findsOneWidget);
-      expect(find.byIcon(StreamIconData.xmark), findsNothing);
+        expect(find.byType(StreamButton), findsOneWidget);
+        expect(find.byIcon(StreamIconData.arrowLeft), findsOneWidget);
+        expect(find.byIcon(StreamIconData.chevronLeft), findsNothing);
+        expect(find.byIcon(StreamIconData.xmark), findsNothing);
+      } finally {
+        debugDefaultTargetPlatformOverride = null;
+      }
+    });
+
+    testWidgets('inserts back chevron on a regular pushed route on iOS', (tester) async {
+      debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+      try {
+        await tester.pumpWidget(_withStreamTheme(const _LauncherScreen()));
+        await tester.tap(find.text('Open'));
+        await tester.pumpAndSettle();
+
+        expect(find.byType(StreamButton), findsOneWidget);
+        expect(find.byIcon(StreamIconData.chevronLeft), findsOneWidget);
+        expect(find.byIcon(StreamIconData.arrowLeft), findsNothing);
+        expect(find.byIcon(StreamIconData.xmark), findsNothing);
+      } finally {
+        debugDefaultTargetPlatformOverride = null;
+      }
     });
 
     testWidgets('inserts cross icon on a fullscreen dialog', (tester) async {
@@ -85,8 +123,8 @@ void main() {
     );
 
     testWidgets(
-      'inserts back chevron when a StreamSheetRoute covers another sheet',
-      (tester) async {
+      'inserts back chevron when a StreamSheetRoute covers another sheet on iOS',
+      (tester) => _onPlatform(TargetPlatform.iOS, () async {
         await tester.pumpWidget(_withStreamTheme(const _StreamSheetLauncher()));
         await tester.tap(find.text('Open stream sheet'));
         await tester.pumpAndSettle();
@@ -123,13 +161,13 @@ void main() {
           ),
           findsNothing,
         );
-      },
+      }),
     );
 
     testWidgets(
       'inserts cross at first nested route inside a StreamSheetRoute and '
-      'back chevron at deeper nested routes',
-      (tester) async {
+      'back chevron at deeper nested routes on iOS',
+      (tester) => _onPlatform(TargetPlatform.iOS, () async {
         await tester.pumpWidget(
           _withStreamTheme(const _StreamSheetLauncher(useNestedNavigation: true)),
         );
@@ -146,13 +184,13 @@ void main() {
         // Deeper nested route: back chevron.
         expect(find.byIcon(StreamIconData.chevronLeft), findsOneWidget);
         expect(find.byIcon(StreamIconData.xmark), findsNothing);
-      },
+      }),
     );
 
     testWidgets(
       'inserts back chevron on the first nested route of a stacked sheet '
-      '(stacked + nested combo)',
-      (tester) async {
+      '(stacked + nested combo) on iOS',
+      (tester) => _onPlatform(TargetPlatform.iOS, () async {
         await tester.pumpWidget(
           _withStreamTheme(const _StreamSheetLauncher()),
         );
@@ -194,7 +232,7 @@ void main() {
           ),
           findsNothing,
         );
-      },
+      }),
     );
   });
 
@@ -218,7 +256,7 @@ void main() {
     testWidgets(
       'tapping the chevron on a stacked StreamSheetRoute pops only itself; '
       'parent sheet stays mounted',
-      (tester) async {
+      (tester) => _onPlatform(TargetPlatform.iOS, () async {
         await tester.pumpWidget(_withStreamTheme(const _StreamSheetLauncher()));
         await tester.tap(find.text('Open stream sheet'));
         await tester.pumpAndSettle();
@@ -252,13 +290,13 @@ void main() {
         // Only the stacked sheet popped — the parent sheet is still mounted.
         expect(find.text('Stacked'), findsNothing);
         expect(find.text('Sheet'), findsOneWidget);
-      },
+      }),
     );
 
     testWidgets(
       'tapping the chevron on a deeper nested route pops only that '
       'nested level; the sheet stays mounted',
-      (tester) async {
+      (tester) => _onPlatform(TargetPlatform.iOS, () async {
         await tester.pumpWidget(
           _withStreamTheme(const _StreamSheetLauncher(useNestedNavigation: true)),
         );
@@ -280,13 +318,13 @@ void main() {
         expect(find.text('Deeper'), findsNothing);
         expect(find.text('Sheet'), findsOneWidget);
         expect(find.byIcon(StreamIconData.xmark), findsOneWidget);
-      },
+      }),
     );
 
     testWidgets(
       'tapping the chevron on the first nested route of a stacked sheet '
       'pops the entire stacked sheet (not just the nested route)',
-      (tester) async {
+      (tester) => _onPlatform(TargetPlatform.iOS, () async {
         await tester.pumpWidget(_withStreamTheme(const _StreamSheetLauncher()));
         await tester.tap(find.text('Open stream sheet'));
         await tester.pumpAndSettle();
@@ -319,7 +357,7 @@ void main() {
         // The whole stacked sheet popped — parent sheet still mounted.
         expect(find.text('Stacked'), findsNothing);
         expect(find.text('Sheet'), findsOneWidget);
-      },
+      }),
     );
   });
 
@@ -328,7 +366,7 @@ void main() {
       'props.style > theme.style > token defaults (three-level merge)',
       (tester) async {
         const propsPadding = EdgeInsets.all(7);
-        const themeTitleStyle = TextStyle(fontSize: 99, color: Color(0xFF112233));
+        const themeTitleStyle = TextStyle(fontSize: 18, color: Color(0xFF112233));
 
         // Theme provides only `titleTextStyle`. Props provide only
         // `padding`. Other fields must fall through to token defaults.
@@ -341,9 +379,6 @@ void main() {
               ),
               child: Scaffold(
                 body: StreamSheetHeader(
-                  // Skip the SafeArea wrap so the inner Padding is the
-                  // first one under the header — makes the assertion
-                  // below straightforward.
                   primary: false,
                   automaticallyImplyLeading: false,
                   title: const Text('Title'),
@@ -355,16 +390,15 @@ void main() {
           ),
         );
 
-        // Props win for padding.
-        final padding = tester.widget<Padding>(
-          find
-              .descendant(
-                of: find.byType(StreamSheetHeader),
-                matching: find.byType(Padding),
-              )
-              .first,
+        // Props win for padding (the header passes its resolved padding
+        // through to the [StreamHeaderToolbar]'s `padding` property).
+        final toolbar = tester.widget<StreamHeaderToolbar>(
+          find.descendant(
+            of: find.byType(StreamSheetHeader),
+            matching: find.byType(StreamHeaderToolbar),
+          ),
         );
-        expect(padding.padding, equals(propsPadding));
+        expect(toolbar.padding, equals(propsPadding));
 
         // Theme wins for titleTextStyle (props didn't set it).
         final titleStyle = tester