Underline should be continuous across spacing from tracking

[christianrondeau]

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Description

Fixes SixLabors/ImageSharp.Drawing#405

The issue being that the underline (decoration) length did not take into account the actual advance, but instead only considered the font's advance.

Disclaimer: this code was written using AI (Copilot with Claude Opus 4.8), and reviewed by me.

Note that there is a big difference between Chrome's underline and ImageSharp's, but from my understanding, ImageSharp's the correct one because it's based on the post table metrics, and Chrome ignores it. This PR only solves the tracking issue and does not change the underline behavior in other ways.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84%. Comparing base (18925d5) to head (dc19ff4).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #544   +/-   ##
=====================================
  Coverage     84%     84%           
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  Files        332     332           
  Lines      24684   24683    -1     
  Branches    3612    3612           
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+ Hits       20785   20807   +22     
+ Misses      3011    2990   -21     
+ Partials     888     886    -2     

Flag	Coverage Δ
unittests	84% <100%> (+<1%)	⬆️

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[JimBobSquarePants]

Thanks for the PR. I’m going to reject this one because my local branch already solves the same issue in a way that fits my newer rendering/decorations architecture better.

The core idea is correct: decoration length should use the laid-out glyph advance, including tracking, so underlines/overlines/strikeouts remain continuous across tracking space. However, this PR is based on the older render path and passes a single-axis float advance through the glyph rendering API.

My local branch already passes the full layout advance as a Vector2, keeps fallback behavior for glyph metric advance, and integrates with a newer skip-ink/decorations enabled pipeline. I may still port the regression tests from this PR, because they cover the right behavior, but I don’t want to merge this implementation as-is.

[christianrondeau]

Excellent, thanks!

[olivierbelzile]

Hi 👋 — sorry for commenting on a closed PR, but I wanted to flag something I just noticed. This implementation draws a continuous underline under every characters where in Chrome, the underline is not drawn beneath characters that have a descender extending below the baseline (e.g. g, j, p, q, y).
I've attached a screenshot to illustrate the difference. Just wanted to make sure it was on your radar.

Thanks for your work on this! 🙏

[JimBobSquarePants]

See above.

integrates with a newer skip-ink/decorations enabled pipeline

That new behaviour (enabled by default) will match chrome etc. I’ve got it working locally cannot release it without a matching Drawing release which I’m still working on. Not long though!

Interestingly I actually found a bug in Chrome while implementing it. If you set the stroke width past a certain ratio to the font the underline is moved down from the baseline.