fix(browser): Rebase FP/FCP against activationStart for prerendered pages - #23032
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fix(browser): Rebase FP/FCP against activationStart for prerendered pages#23032logaretm wants to merge 1 commit into
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…ages `_trackFpFcp` stored the raw paint `entry.startTime` as the fp/fcp measurement. For a page prerendered via the Speculation Rules API and later activated, those timestamps are relative to the prerender navigation start, so the reported value is inflated by the full time the document sat dormant in the prerender buffer. Unlike LCP and TTFB (which go through the vendored web-vitals library and are already corrected), FP/FCP had no correction anywhere. Rebase against `activationStart` and clamp at 0, matching the vendored library's `Math.max(entry.startTime - getActivationStart(), 0)`. `activationStart` is 0 for regular loads, so non-prerendered pages are unchanged.
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Corrects First Paint and First Contentful Paint values on the pageload span for pages prerendered via the Speculation Rules API.
_trackFpFcpstored the raw paintentry.startTimeas the fp/fcp measurement. When a page is prerendered and later activated, paint timestamps are relative to the prerender navigation start, so the value is inflated by the entire time the document sat dormant in the prerender buffer. LCP and TTFB already go through the vendored web-vitals library and are activation-corrected there, but FP/FCP took the raw paint path and had no correction anywhere.I reproduced this in a real Chrome with an actual Speculation Rules prerender: activationStart came out at ~11300ms, the old code stored fcp=11352ms, and the fixed code reports 51ms, which is the real user-perceived paint time. The ~11.3s gap is exactly the buffer dwell time the old value was carrying.