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feat(browser): Add navigation activationStart timestamp to pageload span - #13658
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AbhiPrasad
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Sep 13, 2024
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In prerendering scenarios, where a page might be prefetched and pre-rendered before the user clicks a link, the performance API starts measuring the navigation earlier than when the user clicks it. Web Vital measurements however are always based on the user-perceived (loading) time, so they are not reported from the actual start of the navigation, but rather from the time where the user actively started the navigation, for example by clicking a link. This user action is called "activation" and the time between navigation start and activation is stored in the
activationStartattribute of the "navigation" PerformanceEntry.This PR adds this
activationStartproperty as it gets reported by the browser asperformance.activationStartspan attribute to the pageload span. The browser gives us a relative time in ms, based on thestartTimeattribute. ThestartTimeattribute is always 0 for navigation entry but in absolute numbers, it corresponds toperformance.timeOrigin(which we already send as a span attribute since #13502). So if necessary, in the product, we can calculate the absolute timestamp viaperformance.timeOrigin + performance.activationStart.We can use this attribute as a heuristic where to draw the vertical web vital lines for TTFB, LCP (and possibly FCP) from.