fix(browser): Source FCP from web-vitals onFCP and rebase FP against activationStart - #23032
Merged
Merged
Conversation
Contributor
size-limit report 📦
|
…t `activationStart` FP and FCP were the only vitals still read straight off the `paint` observer, storing the raw `entry.startTime`. On a page prerendered via the Speculation Rules API, paint timestamps are relative to the prerender navigation start, so both values carried the entire time the document sat dormant in the prerender buffer. FCP now goes through web-vitals' `onFCP` like CLS/LCP/TTFB/INP already do, which applies the `activationStart` correction itself and defers registration until activation. web-vitals has no `onFP`, so FP stays on the paint observer and applies the same correction inline. `onFCP` costs nothing in bundle size: `onCLS` already imports it, and that import is static.
logaretm
force-pushed
the
awad/prerender-fcp-fp-activationstart
branch
from
August 13, 2026 16:14
21daec9 to
b2a1abc
Compare
logaretm
changed the title
onFCP and rebase FP against activationStart
logaretm
marked this pull request as ready for review
August 13, 2026 16:33
logaretm
requested review from
Lms24 and
msonnb
and removed request for
a team
August 13, 2026 16:33
![cursor[bot]](https://avatars.githubusercontent.com/in/1210556?s=60&v=4){kind=small}
There was a problem hiding this comment.
Cursor Bugbot has reviewed your changes and found 2 potential issues.
❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.
Reviewed by Cursor Bugbot for commit b2a1abc. Configure here.
| return addFcpInstrumentationHandler(({ metric }) => { | ||
| _measurements['fcp'] = { value: metric.value, unit: 'millisecond' }; | ||
| }); | ||
| } |
There was a problem hiding this comment.
FCP dropped before prerender activation
High Severity
Sourcing fcp from web-vitals onFCP waits for page activation before reporting. On a Speculation Rules prerender the pageload span often finishes during the hidden prerender (load plus idle timeout), so finalizeWebVitals removes the handler before onFCP runs and First Contentful Paint never reaches the span.
Additional Locations (1)
Reviewed by Cursor Bugbot for commit b2a1abc. Configure here.
Contributor
2 similar comments
Contributor
Contributor
Lms24
approved these changes
Aug 28, 2026
Lms24
left a comment
Lms24
left a comment
Member
There was a problem hiding this comment.
Sorry for the late review! LGTM!
The correction was applied when the paint entry arrived, but on a prerendered page that happens while the document is still in the prerender buffer, before the user has activated it. `activationStart` is 0 at that point, so subtracting it did nothing and FP kept the prerender-relative timestamp. The web-vitals-sourced metrics avoid this because they only start observing after activation. FP comes off a buffered paint observer we register at init, so it can't. Store the raw `startTime` instead and do the subtraction in `addWebVitalsToSpan`, by which point activation has happened. Also rewrites the prerender test, which stubbed a non-zero `activationStart` before emitting any entries and so never covered the prerender window.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Corrects First Paint and First Contentful Paint on the pageload span for pages prerendered via the Speculation Rules API.
Both were read straight off the
paintobserver as a rawentry.startTime, so on a prerendered page they carried the whole time the document sat in the prerender buffer.Rather than patch the arithmetic in two places I moved FCP onto web-vitals'
onFCP, which is where CLS/LCP/TTFB/INP already get theiractivationStartcorrection.web-vitals has no
onFP, so FP stays on the paint observer with the correction applied inline.