Fix SplitPane RangeError when child count changes between rebuilds

packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md

@@ -15,7 +15,10 @@ To learn more about DevTools, check out the
 
 ## General updates
 
-TODO: Remove this section if there are not any updates.
+* Fixed a `RangeError` thrown by `SplitPane` when the parent rebuilt the
+  widget with a different number of children, for example when toggling a
+  panel in or out of the layout. -
+  [#9822](https://github.com/flutter/devtools/pull/9822)
 
 ## Inspector updates
 

packages/devtools_app_shared/CHANGELOG.md

@@ -3,6 +3,10 @@ Copyright 2025 The Flutter Authors
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
 -->
+## 0.5.2-wip
+* Fix a `RangeError` thrown by `SplitPane` when the number of children
+  changes between rebuilds.
+
 ## 0.5.1
 * Add DevTools-styled text field `DevToolsTextField`.
 * Updates `devtools_shared` constraint to `^13.0.0`.

packages/devtools_app_shared/lib/src/ui/split_pane.dart

@@ -88,7 +88,7 @@ final class SplitPane extends StatefulWidget {
 }
 
 final class _SplitPaneState extends State<SplitPane> {
-  late final List<double> fractions;
+  late List<double> fractions;
 
   bool get isHorizontal => widget.axis == Axis.horizontal;
 
@@ -98,6 +98,18 @@ final class _SplitPaneState extends State<SplitPane> {
     fractions = List.of(widget.initialFractions);
   }
 
+  @override
+  void didUpdateWidget(SplitPane oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    // When the number of children changes, the previously stored [fractions]
+    // list will be out of sync with [widget.minSizes] and [widget.children],
+    // which causes a RangeError during layout. Reset to the new
+    // [initialFractions] when the child count changes.
+    if (oldWidget.children.length != widget.children.length) {
+      fractions = List.of(widget.initialFractions);
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return LayoutBuilder(builder: _buildLayout);

packages/devtools_app_shared/pubspec.yaml

@@ -3,7 +3,7 @@
 # found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
 name: devtools_app_shared
 description: Package of Dart & Flutter structures shared between devtools_app and devtools extensions.
-version: 0.5.1
+version: 0.5.2-wip
 repository: https://github.com/flutter/devtools/tree/master/packages/devtools_app_shared
 
 environment:

packages/devtools_app_shared/test/ui/split_pane_test.dart

@@ -1154,6 +1154,67 @@ void main() {
       );
     });
 
+    group('rebuilds with a different number of children', () {
+      testWidgets(
+        'does not throw a RangeError when child count shrinks',
+        (WidgetTester tester) async {
+          final threeChildSplit = buildSplitPane(
+            Axis.horizontal,
+            children: const [_w1, _w2, _w3],
+            initialFractions: const [0.2, 0.4, 0.4],
+            minSizes: const [50.0, 50.0, 50.0],
+          );
+          await tester.pumpWidget(wrap(threeChildSplit));
+          expect(find.byKey(_k1), findsOneWidget);
+          expect(find.byKey(_k2), findsOneWidget);
+          expect(find.byKey(_k3), findsOneWidget);
+
+          final twoChildSplit = buildSplitPane(
+            Axis.horizontal,
+            children: const [_w1, _w2],
+            initialFractions: const [0.5, 0.5],
+            minSizes: const [50.0, 50.0],
+          );
+          await tester.pumpWidget(wrap(twoChildSplit));
+          await tester.pumpAndSettle();
+
+          expect(tester.takeException(), isNull);
+          expect(find.byKey(_k1), findsOneWidget);
+          expect(find.byKey(_k2), findsOneWidget);
+          expect(find.byKey(_k3), findsNothing);
+        },
+      );
+
+      testWidgets(
+        'does not throw a RangeError when child count grows',
+        (WidgetTester tester) async {
+          final twoChildSplit = buildSplitPane(
+            Axis.horizontal,
+            children: const [_w1, _w2],
+            initialFractions: const [0.5, 0.5],
+            minSizes: const [50.0, 50.0],
+          );
+          await tester.pumpWidget(wrap(twoChildSplit));
+          expect(find.byKey(_k1), findsOneWidget);
+          expect(find.byKey(_k2), findsOneWidget);
+
+          final threeChildSplit = buildSplitPane(
+            Axis.horizontal,
+            children: const [_w1, _w2, _w3],
+            initialFractions: const [0.2, 0.4, 0.4],
+            minSizes: const [50.0, 50.0, 50.0],
+          );
+          await tester.pumpWidget(wrap(threeChildSplit));
+          await tester.pumpAndSettle();
+
+          expect(tester.takeException(), isNull);
+          expect(find.byKey(_k1), findsOneWidget);
+          expect(find.byKey(_k2), findsOneWidget);
+          expect(find.byKey(_k3), findsOneWidget);
+        },
+      );
+    });
+
     group('axisFor', () {
       testWidgetsWithWindowSize(
         'return Axis.horizontal',