core(unused-css): clamp used bytes to avoid negative wasted bytes - #17131
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NickNojiri wants to merge 3 commits into
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core(unused-css): clamp used bytes to avoid negative wasted bytes#17131NickNojiri wants to merge 3 commits into
NickNojiri wants to merge 3 commits into
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computeUsage sums (endOffset - startOffset) across every used rule reported by CSS coverage, then derives wasted bytes from totalBytes - usedBytes. Coverage can report overlapping ranges (a nested rule lives inside its parent rule's range) or the same rule more than once, so the summed used bytes can exceed the stylesheet size, producing a negative wastedBytes/wastedPercent. A 0-length (empty inline) stylesheet also made percentUnused NaN. Clamp usedUncompressedBytes to the stylesheet size and guard the zero-length case so usage stays within [0, 100]%. Relates to the nested-rule range-overlap concern in GoogleChrome#14718. Add regression tests for overlapping ranges, duplicate used rules, and empty stylesheets.
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Summary
computeUsage in core/computed/unused-css.js sums each used rule's byte range with no clamping. CSS coverage can report overlapping ranges (a nested rule lives inside its parent's range) or the same rule twice, so summed used bytes can exceed the stylesheet size — producing negative wastedBytes/wastedPercent (verified: -4 for overlapping ranges, -8 for a duplicate rule) and NaN for empty stylesheets.
This is the overlap concern raised in #14718, complementing #17127: the smoke test proves current correctness; this makes the computation robust to a regression. The sibling unused-javascript-summary already avoids this via a per-byte bitmap.
Fix
Clamp usedUncompressedBytes to the stylesheet size and guard the zero-length case, keeping usage within [0, 100]%. Regression tests added for overlapping ranges, duplicate used rules, and empty stylesheets.
Testing