Win32 - Ensure owner topmost flag is set if its topmost when showing a owned window

[emmauss]

What does the pull request do?

If a window with an owner is being shown, and the owner is Topmost, ensure the flags for topmost window is set for that owner.

What is the current behavior?

Topmost flag on owner isn't being respected for owned windows when being shown, making them appear behind the owner.
This is the behavior currently

1. Main Window.Topmost is true
2. Main Window shows, Topmost flag is set
3. Main Window remains topmost
4. Child window is created, shown with main window as owner
5. Child window appears behind main window
6. Main window still has Topmost flag and appears above every window. Child doesn't have topmost exstyle flag

On Wpf, child will appear above main window in 5, and will have the topmost ex style flag in 6, but not the Topmost window flag

What is the updated/expected behavior with this PR?

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

If the owner window is Topmost, the HWND_TOPMOST is set once more before the owned window is shown. This only needs to be set once.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0049221-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. 11.2.999-cibuild0049313-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. 11.2.999-cibuild0049416-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

Seems like integration tests are failing

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0049482-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. 11.2.999-cibuild0049488-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]