TextBox: clear selection when moving the caret horizontally

[rastislavsvoboda]

What does the pull request do?

fix clearing selection on TextBox when left/right arrow key is pressed

What is the current behavior?

when there is selection in TextBox, and
selection starts on left, pressing left arrow key does not clear selection
selection starts on right, pressing right arrow key does not clear selection

What is the updated/expected behavior with this PR?

pressing left/right arrow key clears the selection, moving caret to appropriate side of the previous selection

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

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

Obsoletions / Deprecations

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054996-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

Thank you for your contribution!

Unfortunately, the fix isn't quite right. It changes the caret position unexpectedly for all other cases: select a few chars left-to-right, press the right arrow, and notice that the caret is now incorrectly to the left of the selection (and vice versa for a right-to-left with the left arrow).

Moreover, some matching unit tests would be greatly appreciated.

[MrJul]

LGTM, thank you!

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055024-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]