Correct nested TreeViewItem selection behaviour on pointer release

[TomEdwardsEnscape]

This fixes #21427, in which swiping over nested TreeViewItem containers incorrectly selects items. I found that there are actually two scenarios:

1. Swiping from a parent to a nested item selects the parent
2. Swiping from a nested item to a parent selects the nested item

Both a resolved. The changes can be tested with a mouse by applying this style:

<Style Selector="TreeViewItem">
  <Setter Property="InputElement.IsHoldWithMouseEnabled" Value="True"/>  
</Style>

I tried to write a unit test but found that hit testing isn't supported in the headless unit test application used by other TreeView tests.

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

Fixes #21427

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0067165-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

You can use the existing CompositorTestServices to get real hit testing in unit tests.
I wanted to add them to your branch, but it's protected. See MrJul@4d75761 (disclaimer: AI authored).

[MrJul]

Aside from the missing tests, it works as advertised.

[TomEdwardsEnscape]

I added the tests, but I'm not happy about having to emulate Avalonia's gesture/capture logic in the test. The mouse helper class is too low-level for this purpose.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067603-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!