perf(skia): Reuse shared empty clip SKPath

[MartinZikmund]

Reuse a single shared empty SKPath for the native-element clip path when no frame has been recorded yet, instead of allocating a new SKPath() on every such Draw call.

The returned clip path is borrowed read-only by hosts (the non-empty branches already return shared frame-owned / cached paths, and consumers such as the Win32 RenderingNegativePathReevaluated handler only read it to compute a clip region — never mutate or dispose it), so a single shared instance is safe.

Validation

• Compile: Uno.UI.Skia.csproj (net10.0) builds clean — 0 warnings, 0 errors.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KBAigLEAf3csGwnWE22Yr5

[github-actions]

Claude finished @MartinZikmund's task in 2m 35s —— View job

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Review

• Gather context
• Read changed file (CompositionTarget.Rendering.skia.cs)
• Trace all callers of OnNativePlatformFrameRequested / Draw to verify safety claim
• Verify ApplyClipPath (Win32 consumer) does not mutate the incoming path

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Verdict: LGTM — no issues found.

The optimization is correct on all dimensions I checked:

Mutation safety confirmed. Every consumer treats the returned SKPath as read-only:

• macOS (MacOSWindowHost.cs:116): nativeElementClipPath.IsEmpty ? null : nativeElementClipPath.ToSvgPathData() — pure read.
• Win32 (Win32NativeElementHostingExtension.cs:211): path.Op(_tempPath, SKPathOp.Intersect, _tempPath) — reads path, writes the result into _tempPath. No Dispose/Reset/Rewind on path.
• X11, browser, Linux FB, Android, Apple UIKit renderers: all use the path only for IsEmpty checks or to call ToSvgPathData().

Thread safety confirmed. _emptyPath is static readonly and never written after type-initialization; sharing it across concurrent render threads is safe.

Latent memory-leak also fixed. The previous return new SKPath() (no-frame branch) was never disposed by any caller — an allocation that leaked on every paint tick before the first frame arrived. The shared static eliminates both the per-call allocation and the leak.

Comment is appropriate. Two-line comment explaining the non-obvious thread/mutation invariant is exactly what the code style asks for.

One observation (pre-existing, not introduced here): _lastNativeClipPath and _lastScaledNativeClipPath are also static (line 34/36), meaning the scale cache is shared across all CompositionTarget instances in a multi-window scenario. Not a problem in practice today, but worth noting if multi-window render pipelines diverge in the future.

[unodevops]

🤖 Your WebAssembly Skia Sample App stage site is ready! Visit it here: https://unowasmprstaging.z20.web.core.windows.net/pr-23592/wasm-skia-net9/index.html

[nventive-devops]

The build 220109 found UI Test snapshots differences: skia-linux-screenshots: 57, skia-windows-screenshots: 2348, wasm: 11

Details

• skia-linux-screenshots: 57 changed over 2348

  🚨🚨 Comparison Details (first 20) 🚨🚨

  • Gamepad_CurrentReading.png-dark
  • Gamepad_CurrentReading.png
  • Gamepad_Enumeration.png-dark
  • Gamepad_Enumeration.png
  • ContextRequested.png-dark
  • ContextRequested.png
  • DisplayInformation.png-dark
  • ElementLevelTheme.png-dark
  • ElementLevelTheme.png
  • Focus_FocusVisual_Properties.png-dark
  • ImageIconPage.png-dark
  • Examples.png-dark
  • Examples.png
  • ExpanderColorValidationPage.png-dark
  • ExpanderColorValidationPage.png
  • Buttons.png-dark
  • Buttons.png
  • DropDownButtonPage.png-dark
  • DropDownButtonPage.png
  • ButtonClippingTestsControl.png-dark

• skia-windows-screenshots: 2348 changed over 2366

  🚨🚨 Comparison Details (first 20) 🚨🚨

  • ApplicationViewMode.png
  • ApplicationViewSizing.png
  • AppWindowPositionAndSize.png
  • AppXamlDefinedResources.png-dark
  • AutoBorderStretchwithleftmargin.png-dark
  • AutoSuggestBoxChosenSuggestion.png
  • BadgeNotificationTests.png
  • Basics.png
  • BindableDrawerLayout_ChangePane.png
  • BitmapIcon_Foreground.png-dark
  • Border_Clipped_Change_Property.png
  • Border_CornerRadius.png-dark
  • Border_LinearGradientBrush.png-dark
  • Border_LinearGradientBrush.png
  • ButtonClippingTestsControl.png-dark
  • ButtonClippingTestsControl.png
  • CalendarView_Theming.png-dark
  • CalendarView_Theming.png
  • Circle.png-dark
  • Circle.png

• wasm: 11 changed over 1076

  🚨🚨 Comparison Details (first 20) 🚨🚨

  • SamplesApp.Windows_UI_Xaml_Controls.ListView.ListViewSelectedItems
  • UITests.Shared.Windows_UI_Xaml_Controls.MediaPlayerElement.MediaPlayerElement_Minimal
  • UITests.Windows_UI_Xaml_Media_Animation.ColorAnimation_Background
  • SamplesApp.Wasm.Windows_UI_Xaml_Controls.ListView.ListView_IsSelected
  • UITests.Shared.Windows_UI_Xaml_Controls.MediaPlayerElement.MediaPlayerElement_Ogg_Extension
  • UITests.Windows_UI_Xaml_Controls.CalendarView.CalendarView_Theming
  • UITests.Microsoft_UI_Xaml_Controls.WebView2Tests.WebView2_NavigationProperties
  • UITests.Shared.Microsoft_UI_Xaml_Controls.ExpanderTests.WinUIExpanderPage
  • Uno.UI.Samples.Content.UITests.WebView.WebView_AnchorNavigation
  • SamplesApp.Microsoft_UI_Xaml_Controls.WebView2Tests.WebView2_EnableDevTools
  • UITests.Uno_Web.Http.CookieManagerTests

[unodevops]

⚠️⚠️ The build 220109 has failed on Uno.UI - CI.