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LoAF: firstUIEventTimestamp contains non-INP events #1
Description
firstUIEventTimestamp measures the first user interface event, which may include events that are not considered as “interactions” for the more restrictive Interaction Track of DevTools Perormance panel and INP as these only look at certain events with an interactionId. For example,pointerover, pointerdown, mousedown… etc. could all register as the firstUIEventTimestamp but would not count towards Interactions track/INP which might happen later in the frame (or not at all!).
While I think it’s right to measure these in LoAF , it could be confusing to developers if they are using firstUIEventTimestamp to measure LoAFs with interactions.
Should we add a firstInteractionEventTimestamp?
At the very least we should document this.
Reference: https://x.com/blue2blond/status/1751192272572420563?s=46&t=72NqVYI80BM0T97DHBG8iQ
Activity
[-]LOAF: `firstUIEventTimestamp` contains non-INP events[/-][+]LoAF: `firstUIEventTimestamp` contains non-INP events[/+]anatdagan commentedon Jan 27, 2024
tunetheweb commentedon Jan 29, 2024
@anatdagan could you please explain a little more about how this impacts you? You cannot use
firstUIEventTimestampto calculate RUM INP, and even if you could it wouldn't necessarily reflect the correct INP sincefirstUIEventTimestampmay not be related to the INP event.vanderhoop commentedon Jan 29, 2024
@tunetheweb If this were added (and the what-constitutes-an-interaction logic was consistent between loafs and inp attribution), would mapping a long animation frame to an INP attribution be as simple as
loaf.firstInteractionEventTimestamp === inpAttribution.eventTime? Or would there be additional considerations?It'd be helpful regardless, as I had to poke around quite a bit to find the issues with
firstUIEventTimestampand why it wasn't used in Noam's linking gist.tunetheweb commentedon Jan 29, 2024
You could in theory have two interactions within the same frame, and the INP could be the second one so might not be the
firstInteractionEventTimestamp. Or two interactions that start on the same timestamp.Though struggling to see how, as presumably the render delay part of INP wouldn't be until the end of the frame so the first one should be the longest... 🤔
anatdagan commentedon Jan 29, 2024
mmocny commentedon Jan 30, 2024
As one thing to consider-- LoAF is used for more than just INP measurement. Some folks have been using it for Smoothness and a type of "FPS" measure.
In those cases,
firstUIEventTimestampmight represent the e.g. pointermove/scroll events.I cannot say if the current value is useful or not, but if the issue is INP overlap I think there is a wider discussion about how best to do EventTiming <-> LoAF attribution and we can delay this decision until we gain more experience on this type of attribution work?