Fix border inner rounded clip by wieslawsoltes · Pull Request #20647 · AvaloniaUI/Avalonia

wieslawsoltes · 2026-02-09T10:29:48Z

PR Summary: Child-Only Layout Clipping, Composition Parity, and Samples

What

Add a child-only layout clip API aligned with WPF UIElement.GetLayoutClip.
Implement rounded inner-edge child clipping for Border and ContentPresenter when ClipToBounds is enabled.
Align immediate rendering, composition rendering, and composition hit-testing with child-only layout clips.
Preserve visual rendering while applying child-only clips (background/border unaffected).
Fix composition rendering parity for BackgroundSizing on Border and ContentPresenter.
Add render tests and expected images for layout-clip geometry and ContentPresenter rounded clipping.
Add render tests and baselines for BackgroundSizing parity.
Add a ControlCatalog "Child Clipping" page with rounded child clip, adorner clip, and geometry layout clip examples.

How

Added Visual.GetLayoutClip(Size layoutSlotSize) and TryGetChildClipCore(...) to provide child-only clips.
Stored the arranged layout slot size in Layoutable.Arrange and used it when computing layout clips.
Border and ContentPresenter override TryGetChildClipCore to return a rounded inner-edge RoundedRect using GeometryBuilder.CalculateRoundedCornersRectangleWinUI with BackgroundSizing.InnerBorderEdge.
Immediate renderer now uses the layout clip around child rendering and keeps the visual clip separate.
Composition serialization carries child clip data; server rendering swaps the clip around children only.
Composition hit testing respects layout clips regardless of ClipToBounds and without clamping to visual bounds.
Composition border visuals now serialize BackgroundSizing to match immediate rendering.

Why

Child content should be clipped to rounded inner edges without clipping the control's own background/border.
WPF-aligned GetLayoutClip behavior should be based on layout slot size and independent from ClipToBounds.
Consistent behavior across immediate and composition render paths is required to avoid visual mismatches.

Behavioral Notes

GetLayoutClip affects only children; the visual's own rendering is not clipped.
ClipToBounds = true on Border/ContentPresenter yields rounded inner-edge child clipping.
ClipToBounds = false does not prevent layout clips provided by GetLayoutClip.
Composition and immediate renderers follow the same child-only clip semantics.

Test Coverage

GeometryChildClip_Clips_Child
GeometryChildClip_Clips_When_ClipToBounds_False
GeometryChildClip_Uses_LayoutSlotSize
ContentPresenter_RoundedClip_Clips_Child
Border_BackgroundSizing_OuterBorderEdge
ContentPresenter_BackgroundSizing_OuterBorderEdge
New expected images added for the above render tests.

Sample Coverage

ControlCatalog "Child Clipping" page shows:
- Rounded child clipping for Border.
- Rounded child clipping for ContentPresenter.
- Adorner clipping respecting rounded bounds.
- Geometry layout clip example that works without ClipToBounds.

Fixes #2105

avaloniaui-bot · 2026-02-09T11:28:59Z

You can test this PR using the following package version. 12.0.999-cibuild0062051-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

avaloniaui-bot · 2026-02-09T13:10:08Z

You can test this PR using the following package version. 12.0.999-cibuild0062055-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

robloo · 2026-02-10T19:04:48Z

Awesome! Thanks so much for picking this up. Will definitely dig into the details for my own experience as well.

robloo · 2026-02-10T19:18:41Z

src/Avalonia.Controls/Border.cs

+        bool IVisualWithChildClip.TryGetChildClip(out RoundedRect clip)
+        {
+            var bounds = new Rect(Bounds.Size);
+            var keypoints = GeometryBuilder.CalculateRoundedCornersRectangleWinUI(
+                bounds,
+                LayoutThickness,
+                CornerRadius,
+                BackgroundSizing.InnerBorderEdge);
+            clip = keypoints.ToRoundedRect();
+            return true;
+        }


I know this is still draft so won't comment heavily. But should we try to support ALL geometries rather than just RoundedRect here? If someone creates a star-shaped control and needs to clip children this API won't work.

Note: The geometries can get crazy even in WinUI but I guess theoretically for a border are always some form of RoundedRect.

Note: We also need to support clipping in ContentPresenter

Also note WPF supports arbitrary geometry here as well: https://learn.microsoft.com/en-us/dotnet/api/system.windows.uielement.getlayoutclip

robloo · 2026-02-10T19:23:14Z

src/Avalonia.Controls/BorderVisual.cs

        }
    }

+    public Thickness BorderThickness


Side comment: BackgroundSizing should be added too I think for border visuals to render the same. This is unrelated to the changes here though.

…rounded-clip

avaloniaui-bot · 2026-02-10T20:28:25Z

You can test this PR using the following package version. 12.0.999-cibuild0062115-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

avaloniaui-bot · 2026-02-11T21:35:54Z

You can test this PR using the following package version. 12.0.999-cibuild0062186-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

robloo · 2026-03-06T16:17:23Z

src/Avalonia.Base/Visual.cs

+        /// <summary>
+        /// Attempts to get a child-only clip. By default this uses <see cref="GetLayoutClip"/>.
+        /// </summary>
+        protected virtual bool TryGetChildClipCore(out RoundedRect clip, out Geometry? geometryClip)


If we look at GetLayoutClip based on WPF:

There is no need for anything other than Geometry which covers all cases

The return is nullable which means there isn't a need for true/false and Try semantics

I think these TryGetChildClip methods should be the same... just return an nullable Geometry. Is there really such a performance benefit to justify this non-ideal API?

If the method becomes protected virtual Geometry? GetChildClip() why have both it and the GetLayoutClip() method as well? They are actually redundant.

robloo · 2026-03-06T16:19:22Z

src/Avalonia.Controls/BorderVisual.cs

+        }
+    }
+
+    public BackgroundSizing BackgroundSizing


Should all these inheritdoc ? I'm not sure the convention for visuals like this.

robloo · 2026-03-06T16:19:49Z

src/Avalonia.Controls/BorderVisual.cs

+        protected override void RenderCore(ServerVisualRenderContext ctx, LtrbRect currentTransformedClip)
+        {
+            base.RenderCore(ctx, currentTransformedClip);
+        }


nit: missing newline separating methods

robloo · 2026-03-06T16:19:59Z

src/Avalonia.Controls/BorderVisual.cs

    {
        private CornerRadius _cornerRadius;
+        private Thickness _borderThickness;
+        private BackgroundSizing _backgroundSizing;


nit: missing newline separating fields from method

robloo · 2026-03-20T02:14:23Z

Is there any chance the core avalonia team will have a chance to go over this? I'm worried it's going to miss 12.0. It's a pretty big flaw at this layer of the code though.

Wiesław Šoltés added 6 commits February 9, 2026 11:25

Add child-only clip support for Border

9fcdacd

Add render tests for Border clipping

a2b1776

Update Border sample for clipping examples

45be70d

Merge upstream/master

6fcbdb7

Use PushClipToBounds in composition rendering

8bdeee9

Refactor BorderVisual child clip swap

e8654f5

Fix adorner rounded clip and add render test

65b40f5

robloo mentioned this pull request Feb 10, 2026

Support Clipping to rounded corners #2105

Open

robloo reviewed Feb 10, 2026

View reviewed changes

Merge remote-tracking branch 'upstream/master' into fix/border-inner-…

c662479

…rounded-clip

Wiesław Šoltés added 8 commits February 10, 2026 22:59

Add layout clip API and track slot size

e14a377

Plumb child layout clips through rendering

05dbca2

Add rounded child clipping for Border and ContentPresenter

0381cc5

Add ControlCatalog child clipping samples

17c2186

Add render tests and baselines for child clips

e143822

Sync BackgroundSizing in composition border visuals

eca2032

Add render tests for BackgroundSizing parity

c8d0f00

Merge branch 'master' into fix/border-inner-rounded-clip

72976b7

robloo reviewed Mar 6, 2026

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PR Summary: Child-Only Layout Clipping, Composition Parity, and Samples

What

How

Why

Behavioral Notes

Test Coverage

Sample Coverage

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wieslawsoltes commented Feb 9, 2026 •

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robloo Feb 10, 2026 •

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