Make fullscreen reveal bar non-draggable

[massimopaganigh]

What does the pull request do?

Marks the FullscreenPopover panel in WindowDrawnDecorations as DecorationsElement instead of TitleBar, so the fullscreen reveal bar is no longer registered as a window drag region.

What is the current behavior?

In fullscreen, the normal title bar (PART_TitleBar) is collapsed, so the reveal bar is the only element with ElementRole="TitleBar". Dragging its empty area starts BeginMoveDrag on the fullscreen window, moving it off the fullscreen rectangle and leaving the reveal bar / chrome in a broken state.

What is the updated/expected behavior?

The reveal bar is part of the non-client chrome but is not a drag handle, so a fullscreen window can no longer be moved by dragging it. The caption buttons inside the popover keep their own roles (FullScreenButton, CloseButton) and remain fully functional.

How was the solution implemented (if it's not obvious)?

One-line theme change in
src/Avalonia.Themes.Fluent/Controls/WindowDrawnDecorations.xaml:

-                   WindowDecorationProperties.ElementRole="TitleBar">
+                   WindowDecorationProperties.ElementRole="DecorationsElement">

Breaking changes

None. Behavioral change only: the fullscreen reveal bar is no longer a drag region (it should not have been one).

Fixed issues

Fixes #21547

[massimopaganigh]

#21549

It's not related but let me know if it's a good idea.

[cla-avalonia]

• All contributors have signed the CLA.

[massimopaganigh]

@cla-avalonia agree

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0066257-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0066329-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

Since TitleBar is explicitly documented as:

The element acts as a titlebar drag area

I think the change makes sense.

Ideally, dragging the bar would restore the window to its normal state and continue the drag, matching what happens with a Maximized window on Windows. In practice, that will be quite hard to do correctly in a cross-platform way. Disabling the drag is the better behavior.

[MrJul]

Also apply the change to Themes.Simple.
Edit: done.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069907-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]