AutoSuggestBox shouldn't be sealed, or NavigationView.AutoSuggestBox should allow templatization

[mfeingol]

Describe the bug

AutoSuggestBox is sealed and therefore disallows derived classes. The AutoSuggestBox property on the NavigationView requires an AutoSuggestBox and cannot be templatized. Title bar customization is only enabled on Windows 11. Therefore, an app that supports Windows 10 as well as 11 and wants to provide search functionality in the title bar will likely duplicate their ASB in the title bar and in the NavigationView, and enable one or the other depending on OS support. This means any customization and logic used by the ASB will be duplicated in both locations.

Ideally we'd create a user control containing an ASB, or a derived ASB class. But we can do neither in this case.

Steps to reproduce the bug

Read the documentation

Expected behavior

We should be able to user either a UserControl or a derived ASB class.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.4.3: 1.4.231115000

Windows version

Windows 11 (22H2): Build 22621

Additional context

No response

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Closed similar issues:

• AutoSuggestBox still shows when NavigationView is closed, even when disabled via API (#4097), similarity score: 0.75

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[ranjeshj]

@mfeingol Thanks for filing the issue. Several types are sealed that we would like to unseal - this is a known problem with WinUI tracked here - #780