fix: set WindowDrawnDecorations background to transparent

[ent3m]

What does the pull request do?

Fixes a regression where WindowDrawnDecorations used an opaque background, blocking transparency effects (Mica/Acrylic/Blur) when ExtendClientAreaToDecorationsHint is enabled.

What is the current behavior?

WindowDrawnDecorations renders a solid background, obscuring the window's transparency effects when ExtendClientAreaToDecorationsHint="True".

What is the updated/expected behavior with this PR?

WindowDrawnDecorations backgrounds are now transparent , allowing the window background to show through correctly.

Proof (ExtendClientArea = True):

Theme	Before	After
Fluent
Simple

How was the solution implemented?

• Set WindowDrawnDecorationsContent.Underlay backgrounds to SystemControlTransparentBrush in Avalonia.Themes.Fluent/Controls/WindowDrawnDecorations.xaml.
• Set WindowDrawnDecorationsContent.Underlay backgrounds to Transparent in Avalonia.Themes.Simple/Controls/WindowDrawnDecorations.xaml.
• Verified that title bar hit-testing/dragging remains functional.

Checklist

• Added unit tests (if possible)? (Not feasible for this XAML-only theme fix; verified manually via Control Catalog)
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Fixed issues

Fixes #21082

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065345-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[ent3m]

@cla-avalonia agree

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065371-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

The changes make sense, the window's background will always be on top of the underlay anyway.
LGTM!

[kekekeks]

Was this change actually tested?

This is what happens if you enable client area extension:

[ent3m]

Looks good to me.

[ent3m]

Did you mean to say that the title bar shouldn't be transparent when TransparencyLevelHint is None?

[kekekeks]

1. Win32 isn't the only platform that uses drawn decorations
2. Titlebar can be drawn when client area isn't extended (i. e. when it's NOT covered by Window.Background)

[kekekeks]

Also, titlebar being transparent doesn't match what we have with macOS when client area is extended.

[kekekeks]

We should probably have a flag to disable titlebar background uniformly across platforms in client area extension mode

[ent3m]

Thanks for clarifying. I understand the issue now. WindowDrawnDecorations is used when client area isn't extended due to platforms not supporting native title bars.

Avalonia/src/Avalonia.X11/X11Window.cs

Line 1544 in 9d4c952

private bool UseManagedDecorations => _extendingClientAreaToDecorations || _platform.Options.ForceDrawnDecorationsInternal;

Instead of adding a new flag, how about we make the background brush conditional? We can make it transparent only when ExtendClientAreaToDecorationsHint=True and a transparency effect is active (WindowTransparencyLevel != None). This keeps the public API minimal, protects Linux CSD, and aligns with the macOS behavior when transparency is off. A simple implementation would be:

<Style Selector="^:has-transparency /template/ Border#PART_WindowBorder">
  <Setter Property="Background" Value="{DynamicResource SystemControlTransparentBrush}" />
</Style>

<Style Selector="^:has-transparency /template/ Panel#PART_TitleBar">
  <Setter Property="Background" Value="{DynamicResource SystemControlTransparentBrush}" />
</Style>

I'd be happy to submit another PR if that sounds agreeable.