Main Window flickers in incorrect location prior to rendering in v11.1.0-beta1

[billhenn]

Describe the bug

When a main Window is opened for an app, it can briefly show its titlebar and frame in a certain location, then it moves to another location and displays fully-rendered. This behavior was first observed in v11.1.0-beta1 and @grokys narrowed the change that caused it down to this PR.

To Reproduce

• Follow steps here to download and open our sample solution: https://github.com/Actipro/Avalonia-Controls#download-and-run-samples
• In the /Samples/SampleBrowser/References/Avalonia.References.props file, update AvaloniaVersion to 11.1.0-beta1.
• Do a full solution rebuild.
• Run the app and see if you notice the ghost window flicker before the main window shows properly.
• Revert AvaloniaVersion back to 11.0.7 (or a later 11.0 build) and full rebuild solution and run again. You won't see the flicker.

Expected behavior

There should be no flicker when the main window is opened.

Avalonia version

11.1.0-beta1

OS

Windows

Additional context

This flicker was observed on latest Windows 11.

[grokys]

@TomEdwardsEnscape any idea how #14470 might have caused this, and what the fix might be?

[TomEdwardsEnscape]

Looks like even though SetWindowPos will restore a minimised window, it will not show one that has not yet been shown. This means that a change in behaviour snuck in via #14470 when an unshown window is resized.

The problem is noticeable in the Actipro repo because their window takes a long time to open, then removes window decorations and centres itself on-screen. But it's replicable in any window if those conditions are replicated.

[billhenn]

It looks like #15035 resolves the issue. Thanks!