Nested reflow tracing markers don't work

I encountered a profile today which had a long reflow without a corresponding blue box in the timeline.

It turns out we can have nested reflow flushes: PresShell::DoFlushPendingNotifications -> PresShell::ProcessReflowCommands -> PresShell::DidDoReflow -> PresShell::HandlePostedReflowCallbacks -> PresShell::DoFlushPendingNotifications -> PresShell::ProcessReflowCommands

Then the markers are in the order start, start, end, end, and when we convert them to TracingMarkers we don’t resolve nested markers of the same type correctly.

[julienw]

Thanks panos, this is useful to see that the bug happens more often than we think. This should be one half day, we should try to fix this soon.

[mstange]

@mstange is there any way to fix this in perf.html? How do we know which is the start and end marker that are correctly associated?

Oops, I never answered this question.

For now, I think we should assume that multiple markers of the same name are properly nested, and not half-overlapping. In the future we should add IDs so that we can match them up precisely instead of relying on brittle assumptions like this one.