Search all SelectingItemsControl items with TextSearch on key input, not just realized ones

[jonko0493]

What does the pull request do?

This PR brings the behavior of SelectingItemControls with VirtualizingPanel presenters into parity with those controls without virtualizing presenters.

What is the current behavior?

Currently, SelectingItemsControls with VirtualizingPanel item presenters (such as ComboBox) do not search the full contents of their boxes with text search, only the currently realized controls. This means ~that if a ComboBox is closed, typing does nothing, and even when opened,~ (this was slightly wrong!) it will only jump to items immediately in the popup.

What is the updated/expected behavior with this PR?

Now, ComboBoxes and other SelectingItemsControls with virtualizing presenters will have proper TextSearch for their entire item sets.

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

The anonymous method that existed in the SelectingItemsControl class previously has been moved into a static method in ItemsPresenter. It's now used there and in VirtualizingPanel to check search their items/children for text search. (Note: I don't know if there is a preferred place for a static method like this; happy to move as appropriate.)

Unit test case was added to an existing test and the test was adapted so that ComboBoxes in tests will use a VirtualizingStackPanel like the real controls do to ensure the tests fail without this change.

Checklist

• [x] Added unit tests (if possible)?
• [x] 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

Fixes #12490.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053300-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• [x] All contributors have signed the CLA.

[jonko0493]

@cla-avalonia agree

[jonko0493]

Everything was passing prior to me merging master... not sure what changed, could be transient failures?

[timunie]

@jonko0493 don't worry tests are a bit flaky atm

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053407-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]