DynamicResource for TransparencyBackgroundFallback not working

[StefanKoell]

Describe the bug

As discussed here: #16091

When using a dynamic resource in TransparencyBackgroundFallback on a Window, it doesn't honor the light and dark variant resource based on the actual theme. Switching the theme will leave you stuck with the initial value.

To Reproduce

Create an empty project and set the following properties on a Window:

    TransparencyBackgroundFallback="{DynamicResource ThemeBackgroundBrush}"
    TransparencyLevelHint="Mica,Transparent"

Toggling the theme during runtime when Mica is not available does not use the ThemeBackgroundBrush for the current theme.

Expected behavior

Toggling the theme during runtime when Mica is not available should use the ThemeBackgroundBrush for the current theme.

Avalonia version

11.1.0-rc1

OS

Windows

Additional context

No response

[maxkatz6]

This property is read only in this event handled

Avalonia/src/Avalonia.Controls/TopLevel.cs

Lines 746 to 753 in 143399f

	if (transparencyLevel == WindowTransparencyLevel.None)
	{
	_transparencyFallbackBorder.Background = TransparencyBackgroundFallback;
	}
	else
	{
	_transparencyFallbackBorder.Background = null;
	}

, when TransparencyLevel was changed. There is no subscription on property itself changing.

[maxkatz6]

Possibly, 748 line should be replaced with a .Bind call.

[StefanKoell]

Thanks for the feedback! I'm happy to submit a PR but I'm not sure what the right way would be to improve the code. Do I just call the Bind method for that property? Do I need to take care of disposal of the binding?