Given_TextBlock.When_Inlines_Transitively_Change fails when Windows is in dark mode (expects OS-theme default brush) #25013

Current behavior 🐛

Given_TextBlock.When_Inlines_Transitively_Change (runtime test, Skia Desktop) fails deterministically on a Windows machine whose app mode is dark (AppsUseLightTheme = 0). CI agents use the light theme, so CI passes and the failure is only seen locally:

Expected 'FFFFFFFF' in rectangle '{X=0,Y=0,Width=88,Height=19}', but no pixel has this color.
The closest pixel found is '00000000' at '0,0' with a (exclusive) difference of 255.

The text is rendered, but in the theme the test host uses (Light, dark-grey #E4000000 glyph pixels), not in the color the test expects. The test asserts against Uno.UI.Xaml.Media.DefaultBrushes.TextForegroundBrush, which resolves to the OS theme (white on a dark OS), while the TextBlock's real foreground follows the app/element theme (Light).

Diagnostic output from the failing run (temporary logging added to the test):

DefaultBrushes.TextForegroundBrush = #FFFFFFFF
SUT.ActualTheme                    = Light
SUT.Foreground                     = #E4000000
bitmap colors                      = #00000000 (1329 px), #E4000000 (197 px), plus anti-aliasing shades of #xx000000

Expected behavior 🎯

The test should pass regardless of the developer machine's OS theme. It should either force a theme on the TextBlock (RequestedTheme) or compare against the foreground the control actually resolved (SUT.Foreground) instead of the OS-theme-derived DefaultBrushes.TextForegroundBrush.

How to reproduce it (as minimally and precisely as possible) 🔬

1. On Windows, set Settings > Personalization > Colors > Choose your mode to Dark.
2. Build the SamplesApp (dotnet build src/SamplesApp/SamplesApp/SamplesApp.csproj -c Release -f net10.0-desktop -p:UnoFastDevBuild=true -p:UnoTargetFrameworkOverride=net10.0).
3. Set UITEST_RUNTIME_TESTS_FILTER to the base64 of Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls.Given_TextBlock.When_Inlines_Transitively_Change.
4. From bin/Release/net10.0-desktop run dotnet SamplesApp.dll --runtime-tests=test-results.xml.
5. The test fails with the message above. With the OS in light mode it passes.

Workaround 🛠️

Switch Windows to light app mode before running the runtime tests locally.

Works on UWP/WinUI

Not applicable (Uno-only runtime test and internal helper).

Renderer 🎨

Skia

Affected platforms 📱💻🖥️

Skia Desktop (Win32). Likely any desktop head where the OS reports a dark theme. Not tested elsewhere.

Uno.Sdk version (and other relevant versions) 📦

master bfc8dffd7fe9062160edbc7562f34955e88f57b2, built for net10.0-desktop, Windows 11.

IDE version 🧑‍💻

N/A (command line)

Anything else we need to know? 💬

Confirmed by a real runtime run (see diagnostics above), not just by inspection.

• Test: Given_TextBlock.cs#L735-L753 uses DefaultBrushes.TextForegroundBrush as the expected color and never sets a theme.
• DefaultBrushes.cs caches an application-level brush (with a CoreApplication.RequestedTheme fallback), which is not necessarily what an individual element renders with.
• Another assertion at line 266 of the same file uses the same helper and may be affected the same way (not checked).

Suggested fix: compare against ((SolidColorBrush)SUT.Foreground).Color, or set SUT.RequestedTheme explicitly before taking the screenshot.