Internal PointerCaptureChanging event

[miloush]

What does the pull request do?

Tracks the source of capture changes and introduces internal PointerCaptureChanging event that gives controls ability to suppress capture change. This allows to implement different mouse capture semantics, notably suppressing platform capture loss.

What is the current behavior?

Implicit capture model (Linux+macOS) is enforced on all platforms.

What is the updated/expected behavior with this PR?

No behavior changes for Avalonia projects as no control handles this event.

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

The Pointer.Capture will return early if the PointerCaptureChanging event is marked as handled and will keep the existing Captured element.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

No breaking changes expected.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0059364-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0059367-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[Gillibald]

Looks like integration tests for win32 interop are failing now

[miloush]

Right, so there was actually a change, the Implicit captures are now not announced to platforms, which seems to have effect on Windows (I am guessing Avalonia did not feel the need to capture mouse due to the implicit capture). I can change the condition to be != External instead of == Explicit but I want to retest that it still works as it should.