fix(desktop): damage rect rounding + debug overlay scale (#195) - #201
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Finding 2: trackBoundaryDamage scissor rect used truncate(min) + round-half-up(size), which under-covers by 1px at fractional device scales (1.25x, 1.5x, 1.75x). Use Floor(min)/Ceil(max) independently, matching gg's own Context.trackDamage rounding. Extracted into physicalDamageRect helper for independent testing. Finding 3: dirtyOverlay.draw pre-multiplied coordinates by DeviceScale before passing to Fill/Stroke, which apply deviceMatrix again = double scale (4x on 2x display). Removed pre-multiplication, added Push/Identity/Pop state isolation, and border guard for degenerate rects. 9 new tests covering fractional scales, paint state isolation, and degenerate rects.
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Summary
Fixes findings 2 and 3 from #195 (damage rects: logical vs physical pixel mismatches).
Finding 2 (production): scissor rounding
trackBoundaryDamageused truncate+round-half-up-size formula. At fractional scales (1.25x, 1.5x, 1.75x), this under-covers by 1 physical pixel — stale LoadOpLoad seam. Fix:math.Floor/math.Ceilrounding via extractedphysicalDamageRecthelper.Finding 3 (debug overlay): deviceScale²
debug_dirty.gopre-multiplied by scale beforeFill/Strokewhich appliesdeviceMatrixagain = double scale. Fix: removed manual pre-multiplication, added Push/Identity/Pop state isolation.Tests
physicalDamageRecttests (1x, 2x, 1.5x discriminating, 1.25x, 1.75x)Findings 1, 4, 5 from #195 are gg-side — tracked separately.
Test plan
go build ./...— passgo test ./... -count=1— 0 failuresgolangci-lint run --timeout=5m— 0 issues