[ScrollViewerPresenter] Fix rtl scroll (#16667)

[ijklam]

What does the pull request do?

Make ScrollViewerPresenter horizontal scroll more natural when FlowDirection set to RightToLeft.

Fix #16667.

What is the current behavior?

When FlowDirection set to RightToLeft, swiping from right to left will move the left-hand-side content to the right.

default.mp4

What is the updated/expected behavior with this PR?

When FlowDirection set to RightToLeft, swiping from right to left should move the right-hand-side content to the left.

default.mp4

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

Fixed issues

Fix #16667.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053757-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[ijklam]

@maxkatz6 Any updates?

[MrJul]

Thank you for your contribution!
A couple of minor comments and this will be good to merge.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054299-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

This should be done unconditionally, not in an else block. Otherwise, shift + scroll wheel is still reversed.

[ijklam]

I am not sure about this. I think shift+scroll should remained the original behavior, as scrolling up means to show later content. Maybe ask some native Arabic speakers' opinion like @Mop1398

[MrJul]

Afaict, at least on browsers, shift+wheel up scrolls left even with direction: rtl.
But I'm no expert here, let's wait for a native speaker's input.