fix(android): Avoid white flash on first frame

[MartinZikmund]

Avoids the white flash between Android splash-screen dismissal and the first Skia paint.

The splash is held (OnPreDraw keeps returning false) until the GL thread has flushed the first Skia frame. A volatile bool FirstFrameRendered is set on the GL thread after the first _context.Flush(), which posts an invalidation so OnPreDraw re-evaluates and dismisses the splash only once pixels are actually ready.

Validation

• Compile: Uno.UI.Runtime.Skia.Android.csproj (net10.0-android) builds clean — 0 errors (pre-existing XA0101 @(Content) warning only).
• Runtime: pending device / CI.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KBAigLEAf3csGwnWE22Yr5

[github-actions]

Claude finished @MartinZikmund's task in 2m 49s —— View job

---

PR Review: fix(android): Avoid white flash on first frame

• Gather context and understand request
• Read changed files
• Review code for correctness, threading, and style
• Post inline comments
• Post summary

---

Summary

Verdict: fix-first — 1 blocking issue.

The mechanism is correct for the OpenGL path: a volatile bool FirstFrameRendered written on the GL thread after _context.Flush(), with a Post(Invalidate) to wake the main thread so OnPreDraw re-evaluates and dismisses the splash. The volatile write/read pairing across GL and UI threads is sound.

Blocking (1)

• Vulkan path not covered. UnoSKVulkanView.RenderFrame() renders frames but never sets ApplicationActivity.Instance.FirstFrameRendered = true. With FeatureConfiguration.Rendering.UseVulkanOnSkiaAndroid = true, the splash screen will be held indefinitely — effectively a frozen-UI regression on Vulkan devices. The same signaling block from UnoSKCanvasView (set flag → Post(Invalidate)) needs to be placed after _vulkanContext.RenderFrame(...) in UnoSKVulkanView. (inline comment on UnoSKCanvasView.cs:220)

Informational (no action required)

• The XML doc comment on the internal volatile bool FirstFrameRendered field is a bit verbose for an internal field per the repo's comment style (prefer a short single-line comment explaining the non-obvious why). Not a blocker.
• The _contentViewAttachedToWindow flag is only set/read on the main thread; FirstFrameRendered is written on the render thread and read on the main thread. The volatile qualifier is the correct and sufficient synchronization primitive here.
• The RelativeLayout?.Post(() => RelativeLayout?.Invalidate()) double null-check on the static field is defensive and harmless.

Runtime validation pending — as noted in the PR description, device/CI confirmation is still outstanding.

[github-actions]

The equivalent signaling is missing from UnoSKVulkanView.RenderFrame(). When FeatureConfiguration.Rendering.UseVulkanOnSkiaAndroid = true, the Vulkan renderer renders frames but never sets FirstFrameRendered = true, so OnPreDraw() keeps returning false and the splash screen is held forever.

The same block (set flag → post Invalidate) should be placed after _vulkanContext.RenderFrame(...) in UnoSKVulkanView.RenderFrame().

Fix this →

[unodevops]

🤖 Your WebAssembly Skia Sample App stage site is ready! Visit it here: https://unowasmprstaging.z20.web.core.windows.net/pr-23593/wasm-skia-net9/index.html

[nventive-devops]

The build 220110 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) 🚨🚨

  • ContextRequested.png-dark
  • ContextRequested.png
  • DisplayInformation.png-dark
  • ButtonClippingTestsControl.png-dark
  • ButtonClippingTestsControl.png
  • ClipboardTests.png-dark
  • DropDownButtonPage.png-dark
  • DropDownButtonPage.png
  • Focus_FocusVisual_Properties.png-dark
  • ElementLevelTheme.png-dark
  • ElementLevelTheme.png
  • Buttons.png-dark
  • Buttons.png
  • CalendarView_Theming.png-dark
  • CalendarView_Theming.png
  • Examples.png-dark
  • Examples.png
  • ExpanderColorValidationPage.png-dark
  • ExpanderColorValidationPage.png
  • Gamepad_CurrentReading.png-dark

• skia-windows-screenshots: 2348 changed over 2366

  🚨🚨 Comparison Details (first 20) 🚨🚨

  • Accessibility_ScreenReader.png-dark
  • AppBarToggleButtonTest.png-dark
  • AppBar_KeyBoard.png-dark
  • AndroidWindowInsets.png-dark
  • AnimatedIconPage.png-dark
  • AppBarButtonWithIconTest.png-dark
  • AndroidWindowInsets.png
  • AppXamlDefinedResources.png
  • Attributed_text_Supserscript.png-dark
  • AppWindowFeatures.png
  • AppWindowTitleBar Properties.png-dark
  • ApplicationViewSpanningRectsPage.png-dark
  • Arrange_Performance01.png
  • ApplicationViewSizing.png-dark
  • AutoBorderStretchwithtopmargin.png-dark
  • AutoSizedTopLeft.png-dark
  • AppWindowTitleBar Properties.png
  • Attributed_text_Simple.png
  • Attributed_text_Supserscript.png
  • AutoBorderStretchwithleftmargin.png-dark

• wasm: 11 changed over 1076

  🚨🚨 Comparison Details (first 20) 🚨🚨

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