INP - Confused about presentation delay trigger

First: "Paint" is a main thread concept while "Presentation" comes off thread: Compositor Thread -> Gpu Process -> Hardware.  The whole rendering pipeline is parallelized so you can have multiple Paints "in flight" before a previous one is actually presented.  Depending on the platform and browser settings the depth of this pipeline can be limited, but I think ~3-4 Paints is a common limit for frames "in flight".

I don't fully trust the color codings on the Screenshots/Frames layer, but just quickly glancing at the colors in your trace having a bunch of yellow/red signals that the compositor is producing updates without main thread content and so this is a first sign that main thread updates are delayed.

When I switch to Perfetto, I can look at the full details of PipelineReporter trace events, which describe the rendering pipeline.  In this case, I see a very complicated scenario:

• First, from the CrRendererMain thread, I marked the click event processing range, and the Paint + Commit of that "next paint" animation frame
• I then scroll down to find "EventsInAnimationFrame" to find the interaction and look up its EventPresentation time and mark that-- this is the "end" of the interaction.
• With these time points marked, I scroll down to find PipelineReporter trace events which describe what happened with rendering in this time range.
• The PipelineReporter (PR) for the first animation frame which includes the "Next Paint" for the Click event (Look for SendBeginMainFrameToCommit overlapping your Paint), indeed ends with STATE_DROPPED
• Let's call this PR1
• The GPU process does present a STATE_PRESENTED_PARTIAL compositor update for PR1 (it's a different PR event but ends at the same time).
• I don't see any attempt to update rendering for the next 2 vsyncs (assuming 60hz)
• Then there is another main thread Paint+Commit, also DROPPED, also with PARTIAL compositor update (Let's call this PR2)
• Finally, your initial paint is included and presented in a PR3, after approximately 100ms of extra delay after the first PARTIAL presentation

I see two clues:

• Several Animations start before the Click interaction, and a lot start after.  This is a classic sign of "hover" effects being left on mobile, which trigger with touchstart.
• I see a lot of "RunTask" in CompositorGpuThread in this time range

• Chrome Canary if you can
• DevTools experimental feature enabled-- Performance Panel: Show all events