Win32 - set internal _shown flag if ShowWindow will make window visible

[emmauss]

What does the pull request do?

Fixes an issue where, if a window starts out with window state Maximized, the _shown flag isn't set because Windows doesn't send the WM_SHOWWINDOW message for Maximize state. This flag is used internal for resize operations, and should be properly synced.

What is the current behavior?

What is the updated/expected behavior with this PR?

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

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

[TomEdwardsEnscape]

This solution leaves gaps. A window can be shown without hitting the method containing the new code, either because the window was directly manipulated by non-Avalonia code, or because another Avalonia type is showing it (e.g. popups).

There must be a message which is sent to the window when this situation occurs, otherwise it wouldn't know to start repainting. We should identify it and set the bool when that message is received.

[emmauss]

This solution leaves gaps. A window can be shown without hitting the method containing the new code, either because the window was directly manipulated by non-Avalonia code, or because another Avalonia type is showing it (e.g. popups).

There must be a message which is sent to the window when this situation occurs, otherwise it wouldn't know to start repainting. We should identify it and set the bool when that message is received.

We cannot account for changes users make to the window using native apis. As said in the pr description, WM_SHOWWINDOW message isn't sent when the show command is Maximize or Minimize. As for popups, they always shown with ShowNoActivate command, which will send the message.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0049083-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

The proper message to use here is probably WM_WINDOWPOSCHANGED.

The lParam will point to a WINDOWPOS which contains SWP_SHOWWINDOW on show, SWP_HIDEWINDOW on hide, and neither for other position changes.

[emmauss]

The proper message to use here is probably WM_WINDOWPOSCHANGED.

The lParam will point to a WINDOWPOS which contains SWP_SHOWWINDOW on show, SWP_HIDEWINDOW on hide, and neither for other position changes.

This works quite well. Updated to use it.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0049091-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]