[v12] [Regression] `ExtendClientAreaToDecorationsHint` Breaks Transparency/Acrylic Blur/Mica Due To `WindowDrawnDecorations`

[LabrynthKing]

Describe the bug

In v12, TransparencyLevelHint="Transparent", does not seem to make the window transparent and instead produces a black background. AcrylicBlur, Blur, Mica seem to also reproduce the same issue where the background is black.

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:TestProject.ViewModels"
        x:Class="TestProject.Views.Test2Window"
        Width="400" Height="400"
        Title="Test2Window"
        Background="Transparent"
        TransparencyLevelHint="Transparent"
        WindowDecorations="None"
        WindowStartupLocation="CenterScreen"
        CanResize="False"
        CanMaximize="False"
        CanMinimize="False"
        Topmost="True"
        ExtendClientAreaToDecorationsHint="True">

I have also tried just not mentioning background but it reproduces the same issue

To Reproduce

1. Be On Avalonia 12.0.0-rc2
2. Make A New Window
3. Use TransparencyLevelHint="Transparent" In The Window Or Mica/Blur/AcrylicBlur etc.
4. The Result Is A Black Background

Expected behavior

The Window Should Be Transparent As In Example 1 From Avalonia 11.3.13

However, The Window Is Black In Avalonia 12.0.0-rc2

Avalonia version

12.0.0-rc2

OS

Windows

Additional context

OS: Windows 11 25H2 (Build 26200.8037)
Avalonia: 12.0.0-rc2
Last Working Version (According To My Testing): 11.3.13
.NET: 10.0.201 SDK

[d2phap]

Having the same issue since v12.0.0-rc1.
Not only the transparency, but the animation when the window is opened is also lost. To reproduce, we need to remove the window frame:

ExtendClientAreaToDecorationsHint = true;
WindowDecorations = WindowDecorations.BorderOnly;

Note that, it works fine with WindowState = WindowState.FullScreen

[shine1258]

WindowDrawnDecorations will be used when ExtendClientAreaToDecorationsHint = true
https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Themes.Fluent/Controls/WindowDrawnDecorations.xaml
So you need to set Background="Transparent" for PART_WindowBorder and PART_TitleBar in App.Styles

          <WindowDrawnDecorationsContent.Underlay>
            <Panel x:Name="PART_UnderlayWrapper">
              <Border x:Name="PART_WindowBorder"
                      Background="Transparent"
                      BorderThickness="{TemplateBinding FrameThickness}"
                      BorderBrush="{DynamicResource SystemControlForegroundBaseMediumBrush}"
                      IsHitTestVisible="False" />
              <!-- Titlebar: background, title text, and drag area live in underlay -->
              <Panel x:Name="PART_TitleBar" VerticalAlignment="Top"
                     Height="{TemplateBinding TitleBarHeight}"
                     Background="Transparent"
                     IsVisible="{TemplateBinding HasTitleBar}"
                     WindowDecorationProperties.ElementRole="TitleBar" />
            </Panel>
          </WindowDrawnDecorationsContent.Underlay>

I believe that PART_WindowBorder and PART_TitleBar should not have a background by default.

[LabrynthKing]

I believe so too, PART_WindowBorder and PART_TitleBar should not have backgrounds. As if someone wants to set the window transparent or even translucent etc. It should be default to turn into the target instead of changing App.Styles. Just like how it used to work in 11.3.13

[peaceshi]

ExtendClientAreaToDecorationsHint break the Window Mica and AcrylicBlur too.

[atticus-lv]

have the same issue with Breaking the Window Mica

[h2oil]

+1

[ent3m]

I believe that this is caused by a styling issue. I'm working on a fix and will submit a PR soon.