Add ClipContent property to clip children to element bounds

Adds LayoutConfig.ClipContent which emits scissor commands to clip
overflowing children without requiring a scroll container. Exposed
via LayoutStyle for BeginHorizontal/BeginVertical, and applied to
docked window containers and the viewport overlay box.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: andreakarasho

src/Clay.GameEditor/Program.cs

@@ -620,7 +620,8 @@ void RenderViewportContent()
     {
         Padding = Padding.All(20),
         BackgroundColor = Color.Rgba(0, 0, 0, 80),
-        CornerRadius = CornerRadius.All(8)
+        CornerRadius = CornerRadius.All(8),
+        ClipContent = true
     });
 
     if (isPlaying)

src/Clay/ClayUI.cs

@@ -1679,6 +1679,7 @@ public static void BeginHorizontal(ushort gap = 8, ChildAlignment alignment = de
                     ChildGap = gap,
                     ChildAlignment = alignment,
                     Padding = hasStyle ? s.Padding : default,
+                    ClipContent = hasStyle && s.ClipContent,
                     Sizing = new Sizing
                     {
                         Width = hasSizingW ? s.Sizing.Width : SizingAxis.Grow(),
@@ -1792,6 +1793,7 @@ public static void BeginVertical(ushort gap = 8, ChildAlignment alignment = defa
                     ChildGap = gap,
                     ChildAlignment = alignment,
                     Padding = hasStyle ? s.Padding : default,
+                    ClipContent = hasStyle && s.ClipContent,
                     Sizing = new Sizing
                     {
                         Width = hasSizingW ? s.Sizing.Width : SizingAxis.Grow(),
@@ -2139,7 +2141,8 @@ public static bool BeginWindow(string title, ref bool open, WindowStyle? style =
                     Layout = new LayoutConfig
                     {
                         Direction = LayoutDirection.TopToBottom,
-                        Sizing = Sizing.FixedSize(leafData.BoundingBox.Width, leafData.BoundingBox.Height)
+                        Sizing = Sizing.FixedSize(leafData.BoundingBox.Width, leafData.BoundingBox.Height),
+                        ClipContent = true
                     },
                     Floating = new FloatingConfig
                     {
@@ -6371,6 +6374,10 @@ public LayoutStyle() { }
     public BorderConfig Border { get; set; } = default;
     public Padding Padding { get; set; } = default;
     public Sizing Sizing { get; set; } = default;
+    /// <summary>
+    /// When true, children are clipped to this element's bounding box.
+    /// </summary>
+    public bool ClipContent { get; set; } = false;
 }
 
 public struct ScrollAreaStyle

src/Clay/Core/ClayContext.cs

@@ -327,13 +327,20 @@ public void ConfigureOpenElement(ElementDeclaration declaration)
             GenerateIdForAnonymousElement(ref openLayoutElement);
         }
 
+        // ClipContent adds to clip stack (without scroll behavior)
+        if (declaration.Layout.ClipContent)
+        {
+            OpenClipElementStack.Add((int)openLayoutElement.Id);
+        }
+
         // Process scroll config
         if (declaration.Scroll.IsScrollable)
         {
             int scrollIndex = ScrollElementConfigs.Add(declaration.Scroll);
             ElementConfigs.Add(new ElementConfig { Type = ElementConfigType.Scroll, ConfigIndex = scrollIndex });
             openLayoutElement.ElementConfigsLength++;
-            OpenClipElementStack.Add((int)openLayoutElement.Id);
+            if (!declaration.Layout.ClipContent) // Avoid double-push
+                OpenClipElementStack.Add((int)openLayoutElement.Id);
 
             // Find or create scroll container data
             bool found = false;
@@ -384,6 +391,12 @@ public void CloseElement()
         bool elementHasScrollHorizontal = false;
         bool elementHasScrollVertical = false;
 
+        // Pop clip stack for ClipContent elements
+        if (layoutConfig.ClipContent)
+        {
+            OpenClipElementStack.Length--;
+        }
+
         // Check for scroll config
         for (int i = 0; i < openLayoutElement.ElementConfigsLength; i++)
         {
@@ -393,7 +406,8 @@ public void CloseElement()
                 ref var scrollConfig = ref ScrollElementConfigs[config.ConfigIndex];
                 elementHasScrollHorizontal = scrollConfig.Horizontal;
                 elementHasScrollVertical = scrollConfig.Vertical;
-                OpenClipElementStack.Length--;
+                if (!layoutConfig.ClipContent) // Avoid double-pop
+                    OpenClipElementStack.Length--;
                 break;
             }
         }
@@ -1134,6 +1148,8 @@ private void GenerateRenderCommandsRecursive(int elementIndex, short zIndex, Bou
         // Find configs
         int sharedIndex = FindConfigIndex(ref element, ElementConfigType.Shared);
         int scrollIndex = FindConfigIndex(ref element, ElementConfigType.Scroll);
+        ref var layoutConfig = ref LayoutConfigs[element.LayoutConfigIndex];
+        bool clipContent = layoutConfig.ClipContent;
 
         SharedElementConfig sharedConfig = sharedIndex >= 0 ? SharedElementConfigs[sharedIndex] : default;
 
@@ -1154,28 +1170,34 @@ private void GenerateRenderCommandsRecursive(int elementIndex, short zIndex, Bou
             });
         }
 
-        // Scissor start
-        if (scrollIndex >= 0)
+        // Scissor start (for scroll containers or ClipContent elements)
+        bool needsScissor = scrollIndex >= 0 || clipContent;
+        if (needsScissor)
         {
-            ref var scrollConfig = ref ScrollElementConfigs[scrollIndex];
+            ScrollRenderData scrollData = default;
+            if (scrollIndex >= 0)
+            {
+                ref var scrollConfig = ref ScrollElementConfigs[scrollIndex];
+                scrollData = new ScrollRenderData
+                {
+                    Horizontal = scrollConfig.Horizontal,
+                    Vertical = scrollConfig.Vertical
+                };
+            }
             RenderCommands.Add(new RenderCommand
             {
                 BoundingBox = boundingBox,
                 CommandType = RenderCommandType.ScissorStart,
                 Id = element.Id,
                 ZIndex = zIndex,
-                Scroll = new ScrollRenderData
-                {
-                    Horizontal = scrollConfig.Horizontal,
-                    Vertical = scrollConfig.Vertical
-                }
+                Scroll = scrollData
             });
         }
 
-        // If this element is a scroll container, its bounding box clips children
+        // If this element clips children, its bounding box clips children
         var childClip = clipBounds;
         bool childHasClip = hasClip;
-        if (scrollIndex >= 0)
+        if (needsScissor)
         {
             childClip = boundingBox;
             childHasClip = true;
@@ -1238,7 +1260,7 @@ private void GenerateRenderCommandsRecursive(int elementIndex, short zIndex, Bou
         }
 
         // Scissor end
-        if (scrollIndex >= 0)
+        if (needsScissor)
         {
             RenderCommands.Add(new RenderCommand
             {

src/Clay/Layout/LayoutConfig.cs

@@ -34,6 +34,11 @@ public struct LayoutConfig
     /// </summary>
     public LayoutDirection Direction;
 
+    /// <summary>
+    /// When true, children are clipped to this element's bounding box.
+    /// </summary>
+    public bool ClipContent;
+
     public static readonly LayoutConfig Default = new()
     {
         Sizing = Sizing.Default,