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[web] Fix WebParagraph subpixel text stretching and letter jittering - #191030
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Fixes targetRect calculation in WebParagraph painting: - Removes .ceilToDouble() from targetRect width/height, preserving exact sourceRect.width / devicePixelRatio ratio for 1:1 physical pixel mapping without non-uniform scaling artifacts. - Removes .floorToDouble() from targetRect offset coordinates, preserving subpixel positioning and preventing letter jittering. - Removes unnecessary CSS style assignments on offscreen 2D paint canvas. - Adds unit tests covering WebParagraph painting across fractional and integer devicePixelRatio values.
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This pull request updates the web paragraph painter to correctly handle fractional device pixel ratios by removing explicit canvas style width/height resizing and avoiding rounding or flooring of target rectangle coordinates and dimensions. A test is added to verify subpixel precision and correct mapping under fractional DPRs. The review feedback suggests renaming a misleadingly named style variable to improve readability and modifying the mock canvas's fallback method to return null instead of throwing to increase robustness.
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Description
Fixes subpixel text stretching, distortion, and letter jittering issues in
WebParagraphwhen rendered with CanvasKit/Skwasm.Root Cause
targetRectapplied.floorToDouble()to its origin (dx,dy). Snapping logical coordinates to integer pixels while the text canvas was rasterized with floating-point origins caused 1-pixel positioning shifts and letter jittering during smooth scrolling or subpixel placement.targetRectapplied.ceilToDouble()tosourceRect.width / devicePixelRatioandsourceRect.height / devicePixelRatio. On fractional or high-DPI displays (e.g. 2.0 DPR wheresourceRect.width= 21 physical px), rounding the logical width to11.0caused CanvasKit to render the 21 physical pixel source bitmap across 22 physical screen pixels. WithFilterQuality.none, nearest-neighbor sampling stretched the texture, producing distorted glyphs and non-uniform pixel columns._resizePaintCanvasunnecessarily mutated.style.widthand.style.heighton an unattached offscreenDomHTMLCanvasElement.Fix
.ceilToDouble()fromtargetRectdimensions, preserving exactsourceRect.width / devicePixelRatiophysical-to-logical ratio for 1:1 screen pixel mapping..floorToDouble()fromtargetRectorigin coordinates, preserving subpixel positioning accuracy.paragraph_test.dartasserting DPR ratio and subpixel accuracy across fractional and integer DPR values.Tests
test('WebParagraph correctly handles fractional devicePixelRatio')inparagraph_test.dart.Fixes #188321