Fixed TextBlock disappearing when inline used and measure is invalidated

[adirh3]

What does the pull request do?

Fixes an issue that TextBlock with inlines could disappear when it's measure is invalidated.

What is the current behavior?

Invalidating a TextBlock measure with inlines could cause the TextBlock to disappear.
This can be reproduced by going into ControlCatalog/TextBlockPage and then changing Avalonia's theme -

What is the updated/expected behavior with this PR?

The TextBlock should not disappear when it's measure is invalidated.

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

Recalculate _textRuns in the next measure/render after it is set to null by OnMeasureInvalidated.

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-cibuild0054108-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[timunie]

Does it fix #17862 ?

[adirh3]

Does it fix #17862 ?

Checked with their repro, didn't fix it.

[Gillibald]

Please add a failing unit test