Add LineMetrics and per-line measurement API

[JimBobSquarePants]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

This pull request introduces improvements and refactoring to the text layout logic in the SixLabors.Fonts library, focusing on more accurate handling of line spacing and alignment for both horizontal and vertical text layouts. It also adds a new LineMetrics struct to encapsulate line measurement details and exposes additional internal APIs for better testability and extensibility.

Text layout and alignment improvements:

• Refactored line spacing and centering calculations in TextLayout.cs for horizontal, vertical, and mixed vertical layout methods, ensuring glyphs are properly centered within extra space created by LineSpacing. This addresses previous inaccuracies in how line spacing affected glyph placement. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

• Added new internal methods CalculateLineOffsetX and CalculateLineOffsetY to centralize and clarify line alignment logic for both axes, improving maintainability and correctness of text alignment calculations.

API and structure enhancements:

• Added a new LineMetrics struct in LineMetrics.cs to encapsulate measured metrics for a single laid-out text line, supporting both horizontal and vertical layouts.

• Exposed ProcessText as an internal method in TextLayout.cs to facilitate testing and internal usage.

Glyph layout and metric tracking:

• Updated the TextLine class to track an additional metric, ScaledMaxDelta, for more precise line box calculations, and ensured LineSpacing is applied when storing line heights. [1] [2] [3] [4]

Minor fixes and code clarity:

• Cleaned up variable names and comments, and fixed minor logic issues (such as ensuring correct buffer sizing and centering calculations), contributing to overall code clarity and robustness. [1] [2] [3] [4]

Let me know if you want a deeper dive into any of these changes or need help understanding how the new alignment logic works!

[codecov]

Codecov Report

❌ Patch coverage is 66.94915% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 83%. Comparing base (6078cd5) to head (461d46c).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/SixLabors.Fonts/TextLayout.cs	48%	29 Missing and 3 partials ⚠️
src/SixLabors.Fonts/TextMeasurer.cs	83%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #499    +/-   ##
======================================
  Coverage     83%     83%            
======================================
  Files        272     274     +2     
  Lines      18929   19184   +255     
  Branches    2676    2719    +43     
======================================
+ Hits       15836   16081   +245     
- Misses      2422    2426     +4     
- Partials     671     677     +6     

Flag	Coverage Δ
unittests	83% <66%> (+<1%)	⬆️

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[Copilot]

Pull request overview

This PR introduces the LineMetrics struct and a new TextMeasurer.GetLineMetrics() API for per-line measurement. It also refactors line spacing and alignment calculations in TextLayout.cs to correctly apply LineSpacing by baking it into the stored line height at glyph-addition time (rather than multiplying later during layout), and adds CalculateLineOffsetX/Y helper methods for alignment computation.

Changes:

• Adds LineMetrics struct and TextMeasurer.GetLineMetrics(string/ReadOnlySpan<char>, TextOptions) public API
• Refactors line height and alignment calculations in horizontal, vertical, and mixed vertical layout methods to eliminate double-application of LineSpacing
• Adds ScaledMaxDelta tracking to TextLine and GlyphLayoutData to support accurate guide-line positioning in GetLineMetrics

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/SixLabors.Fonts/LineMetrics.cs	New struct encapsulating per-line typographic metrics
src/SixLabors.Fonts/TextMeasurer.cs	New GetLineMetrics overloads; CountLines refactored to use ProcessText
src/SixLabors.Fonts/TextLayout.cs	ProcessText made internal; LineSpacing baked into stored height; CalculateLineOffsetX/Y helpers added; ScaledMaxDelta tracking added
tests/SixLabors.Fonts.Tests/TextLayoutTestUtilities.cs	Added beforeAction/afterAction hooks to TestLayout
tests/SixLabors.Fonts.Tests/Issues/Issues_353.cs	New visual regression test for GetLineMetrics with guide-line drawing
tests/Images/ReferenceOutput/Test_Issue_353_400.png	Reference image for the new test
.github/workflows/build-and-test.yml	Upgraded Codecov action to v5; added required token

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