[WIP]Fix overhang leading calculation for negative values

[Gillibald]

What does the pull request do?

What is the current behavior?

What is the updated/expected behavior with this PR?

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

Fixes: #13671

[MrJul]

I don't think I understand the fix, why isn't it Max(0, bounds.Left)?
If you can spare a few minutes to explain I'd be grateful :)

I assume we don't have tests for this part? This seems easy to accidentally break in the future.

(Edit: may or not be related but recent test TextLineTests.Should_GetPreviousCharacterHit_Non_Trailing doesn't have any assert)

[Gillibald]

Glyph bounds are produced by the current raster backend. For Windows this is DirectWrite for macOS this is CoreText etc.

So glyph bounds might be calculated differently. In the end, we want to know the overhanging black pixels at the left, right, and bottom edges of the regular bounding box.

OverhangLeading is defined as a positive value. It is used to translate rendered text on the x-axis so nothing gets clipped.

We should definitely add a unit test that covers this but writing a test for this involves a custom backend so we get identical glyph bounds on all platforms. We can't write a test that is using Skia.

The mentioned unit tests need to be fixed but are unrelated.