Consider expose the "Hide Win11 corner" function as AttachedProperty

[rabbitism]

So that user can utilize in any kind of window. It is possible to subscribe to the change of IsExtendedIntoWindowDecorationsProperty and apply the coercion. I have tested it locally and it works well.
(And I can open a PR if needed. )

[BAndysc]

I was thinking about an attached property, but I am afraid observing IsExtendedIntoWindowDecorationsProperty is not enough.

Avalonia sets DWMWA_WINDOW_CORNER_PREFERENCE in ExtendClientArea(). ExtendClientArea is called when IsExtendedIntoWindowDecorationsProperty changes, but also in few different cases (ShowWindow, WM_SIZE message, few different cases). So I think there is high risk our call to no rounded corners could be replaced by Avalonia's call breaking the classic style.

I think it is better to just implement it on the Avalonia side: AvaloniaUI/Avalonia#17294

[rabbitism]

I'm suggesting this according to your current implementation. because every call of base.ExtendClientAreaToDecorationsChanged(bool) will set IsExtendedIntoWindowDecoration once (see Avalonia Source ), so eventually there should be no difference.

[BAndysc]

Hmm, how do you use coercion with DirectProperty? I thought only StyledProperty supports it? Tho I think that's still a hack, as coercion should not have side effects (but in this particular case it is not very problematic).

[rabbitism]

Hmm, how do you use coercion with DirectProperty? I thought only StyledProperty supports it? Tho I think that's still a hack, as coercion should not have side effects (but in this particular case it is not very problematic).

Sorry was a bit busy these days. Please check below commit: rabbitism@032a59b

[BAndysc]

This is not enough. This callback will only fire on an effective property value change, i.e. only once, because we don't change IsExtendedIntoWindowDecorationsProperty. On the other hand ExtendClientAreaToDecorations in Window Impl is called more often. I.e. when you minimize and restore the window. In this case the rounded borders fix will be overriden by the default behavior.

So implementing it on the Avalonia side is the only option.