ComboBox on AppWindow Freezes application in WinUI 2

Describe the bug

On versions of WinUI 2 after version 2.6.0-prerelease.210430001, using a ComboBox on an AppWindow will result in the application hanging.

Steps to reproduce the bug

I’ve created a repro solution here: https://github.com/ZodmanPerth/WinUI2ComboBoxFreezeBug. The readme in that repro has replication steps specific to replication on that solution.

For generic reproduction:

1. Create a new UWP app
2. Install WinUI 2 (any version after 2.6.0-prerelease.210430001 replicates the issue)
3. From the main window, spawn a new AppWindow containing a ComboBox with several items
4. Run the app
5. Spawn the AppWindow
6. Open the ComboBox
7. Move your mouse across the open ComboBox, ensuring the mouse goes outside the airspace of the open ComboBox
8. Observe the app hangs

An easy way to observe it hangs is that the ComboBoxItem hover state stops working

Expected behavior

The app should not hang, the UI should remain responsive.

Screenshots

NuGet package version

Microsoft.UI.Xaml 2.8.0-prerelease.220601001

Windows app type

• UWP
• Win32

Device form factor

Desktop

Windows version

Windows 11 (21H2): Build 22000

Additional context

It seems that changes to the style of the ComboBox after 2.6.0-prerelease.210430001 have caused the issue.

[lukeblevins]

Hi @ZodmanPerth.

Just a fellow developer here.

Could you try applying the workaround I mentioned here and let me know if this still repros?

[github-actions[bot]]

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