Fixed indented lines in diff flow direction text

[dme-compunet]

What does the pull request do?

Fixed a bug in text layout causing indented lines in text with reverse flow directions due to incorrect positioning of whitespace characters.

What is the current behavior?

No wrap text:

Wrapped text:

What is the updated/expected behavior with this PR?

No wrap text:

Wrapped text:

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

For no-wrap text:
The default text for a text-run without content is currently 'a', an explicit LTR character. This fix replaces it with a neutral character, enabling the bidi algorithm to consider the direction of preceding RTL characters.

For wrapped text:
In wrapped text, the issue occurs because CoalesceLevels is called only once before performing text wrapping. After this fix, bidi text will be processed again for each line.

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

Fixes #17175

[Gillibald]

Wrapping text should not change the bidi layout. Your changes just do that. So the first bidi layout is wrong or something else is causing this and needs to be fixed. Text seems to be wrapped at the wrong position.

[dme-compunet]

Imagine you have the following text: aaa bbb in RTL paragraph,
If it is a single line then the bidi levels should be like this:

a a a   b b b  
0 0 0 0 0 0 0 1

But after it is broken into two lines, bidi levels should be like this:

a a a   
0 0 0 1
b b b   
0 0 0 1

Is there something wrong in my understanding?

[dme-compunet]

Sorry, you are right, the bidi levels should remain the same,
so if I understand correctly the problem is that this white-space should be set as a trailing-whitespace?

[dme-compunet]

Is commit Set bidi-netural char for default text okay, or does it also need to be thrown away?

[Gillibald]

I think that default text is being used for InlineUIContainer and embedded constrols are always LTR

[dme-compunet]

This is also used for TextEndOfParagraph and causes the preceding whitespaces to be LTR as well