Popup inherits transformation depending on AllowsTransparency

[topsterde]

• .NET Core Version: 5.0
• Windows version: Win 10 1909
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
• Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No

Problem description:
Depending on AllowsTransparency the popup inherits the transformation or not.

Actual behavior:

Expected behavior:

Minimal repro:

<Grid Margin="50" Width="100" Height="100" HorizontalAlignment="Center" VerticalAlignment="Center" Background="LightGray">
        <Grid.LayoutTransform>
            <RotateTransform Angle="45" />
        </Grid.LayoutTransform>
        <Popup IsOpen="True" Placement="Center" AllowsTransparency="False">
            <Border Background="Green">
                <TextBlock Text="Popup without AllowsTransparency"/>
            </Border>
        </Popup>
        <Popup IsOpen="True" Placement="Center" AllowsTransparency="True" >
            <Border Background="Yellow">
                <TextBlock Text="Popup with AllowsTransparency"/>
            </Border>
        </Popup>
    </Grid>

[huiliu-code]

The reason why RotateTransform is not applied is because AllowsTransparency is false. Without this, the Popup has to stay rectangular. If you set it to true the rotation will also be applied. You could use the following method or refer to the method here.

<Grid Margin="50" Width="100" Height="100" HorizontalAlignment="Center" VerticalAlignment="Center" Background="LightGray">
        <Grid.LayoutTransform>
            <RotateTransform Angle="45" />
        </Grid.LayoutTransform>
         <Popup IsOpen="True" Placement="Center" AllowsTransparency="False">
            <Border Background="Green">
                <TextBlock Text="Popup without AllowsTransparency"/>
            </Border>
        </Popup>
        <Popup IsOpen="True" Placement="Center" AllowsTransparency="True" >
            <Popup.LayoutTransform>
                 <RotateTransform Angle="-45" />
            </Popup.LayoutTransform>
            <Border Background="Yellow">
                <TextBlock Text="Popup with AllowsTransparency"/>
            </Border>
        </Popup>
      </Grid>