[WIP]Fix line height calculation for lines that contain fallback fonts

[Gillibald]

What does the pull request do?

This PR changes the line text metrics calculation so fallback fonts do not affect the final metrics.
This also fixes an issue with zero-width glyphs that are being hit-tested.

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

[avaloniaui-bot]

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[kerams]

Oh, cool, I tested this and it actually fixes my line height issues with combined font/script texts, which I thought were caused by some incompatibilities between fonts. #18881 and #18279 will probably see improvement too.

[kerams]

Having said that, the line height reduction also results in diacritics below the descender being clipped

12.0.999-cibuild0056951-alpha vs 12.0.999-cibuild0056532-alpha

[Gillibald]

Could you provide me the string that shows the clipping?

[kerams]

بمكتب محامي

I'll try to whack out a repro project if the problem does not manifest for you (I am not adding any constraints that would cause it that I can see, but maybe the Material theme does).

And it turns out the 2 missing dots are not even combining diacritics, but proper part of the letter https://unicode-explorer.com/c/064A

[avaloniaui-bot]

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[avaloniaui-bot]

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[MrJul]

I don't think that's right, the fallback fonts should be used to determine the line's height as soon as they're used, or we'll get clipping and other issues. They should have no impact if unused though.

Said another way, having FontFamily="B" and FontFamily="A,B" should result in the exact same render if B is the only font effectively used, even if it's a fallback in the latter case.

Example:

<StackPanel TextElement.FontSize="32">
  <TextBlock Background="DodgerBlue" Text="رقم سري" FontFamily="/#Noto Sans Arabic" Margin="8" />
  <TextBlock Background="DodgerBlue" Text="رقم سري" FontFamily="/#Noto Sans,/#Noto Sans Arabic" Margin="8" />
</StackPanel>

Master:

This behavior, which in my opinion is the correct one, was actually a fix introduced in early 11.2 (I think in #15837?). It was broken in 11.1.x.

This PR:

Notice the clipping and line height not being respected.

Fonts: https://fonts.google.com/specimen/Noto+Sans and https://fonts.google.com/specimen/Noto+Sans+Arabic
Noto Sans Arabic is a great testing candidate since it has a huge line height.

[Gillibald]

So there needs to be a different approach for the line height adjustment. We could adjust the scaling of the fallback font to better match the base font. In the end we want to avoid line height changes when a base font is mixed with a fallback font. If there is no ideal solution I need to revert the lineheight related changes.

[Gillibald]

Related: #18438
#18438 (comment)

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You can test this PR using the following package version. 12.0.999-cibuild0057038-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0057041-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]