fix(text): preserve lines at rounded fractional heights - #21834
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A wrapped TextBlock with a fractional line height must retain every line when its desired height lands exactly on a fractional-scale pixel boundary.
TextLayout treated a floating-point accumulation error as a MaxHeight overflow. Use the established tolerant comparison so a line that exactly fits its rounded layout bounds is retained.
NathanDrake2406
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July 23, 2026 16:06
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Floating point errors strike again.
LGTM!
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* test(textblock): cover fractional layout rounding A wrapped TextBlock with a fractional line height must retain every line when its desired height lands exactly on a fractional-scale pixel boundary. * fix(text): preserve lines at rounded fractional heights TextLayout treated a floating-point accumulation error as a MaxHeight overflow. Use the established tolerant comparison so a line that exactly fits its rounded layout bounds is retained. --------- Co-authored-by: Julien Lebosquain <julien@lebosquain.net>
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What does the pull request do?
Fixes #21786 by preventing a wrapped
TextBlockfrom omitting its final line when a fractionalLineHeightlands exactly on a display-scale rounded height.What is the current behavior?
At 125% scaling, three 20.8-unit lines round to 62.4 logical units. Floating-point accumulation can evaluate the total as marginally above 62.4, so
TextLayouttreats the last line as exceedingMaxHeightand omits it.What is the updated/expected behavior with this PR?
All three wrapped lines remain visible when the rounded layout height exactly fits them. Real max-height overflow continues to clip or trim text.
Validation:
TextBlockwithLineHeight=20.8. The test fails before the fix with 2 lines instead of 3.dotnet run --project tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj -- --filter-class "Avalonia.Controls.UnitTests.TextBlockTests"dotnet run --project tests/Avalonia.Skia.UnitTests/Avalonia.Skia.UnitTests.csproj -- --filter-class "Avalonia.Skia.UnitTests.Media.TextFormatting.TextLayoutTests"How was the solution implemented (if it is not obvious)?
The max-height comparison now uses the established tolerant
MathUtilities.GreaterThancheck instead of exact>. This is already used for text-width overflow and avoids treating ordinary floating-point precision error as an extra line.Checklist
Breaking changes
None.
Obsoletions / Deprecations
None.
Fixed issues
Fixes #21786