1908257 - 15+ second janks of Main Process Mainthread when scrolling with mouse wheel

Status: NEW

(Core :: Panning and Zooming, defect, P3)

Description

[Randell Jesup [:jesup] (needinfo me)]

I was interacting with ChatGPT in a tab, and tried to scroll down through a large amount of output it had given. I'm not sure what happened, but after that until I restarted, any attempt to scroll with the mouse wheel would lock the browser for ~17-19 seconds. (See https://share.firefox.dev/3y7jMne)

Page up/down were fine. Closing and switching tabs had no impact.
This was with Nightly Linux, probably from mid-June, on Fedora 39, on a threadripper and Radeon GPU

Comment 1

[Markus Stange [:mstange]]

It's spending a long time in SimulateUntil(aTime);.

aTime comes from mApzc.GetFrameTime().Time(); and it's simulating since mLastSimulatedTime.
So either mLastSimulatedTime is long in the past for some reason, or aTime is far in the future for some reason. I think the former is more likely.

Comment 2

[Dan Robertson (:dlrobertson)]

This sounds a bit like Bug 1877972, but it seems like this issue is in the smooth scroll animation implementation. If this is reasonably reproducible, could you snag a profile with the same logging settings used there? See bug 1877972 comment 2. Since this is a MSD scroll animation issue, this only impacts nightly (see pref definition).

Comment 3

[Botond Ballo [:botond]]

Regardless of the root cause, we may want to have an iteration limit in this loop, since scrolling by an incorrect amount seems like a less bad outcome than hanging while we iterate millions of times.

Comment 4

[BugBot [:suhaib / :marco/ :calixte]]

The severity field is not set for this bug.
:botond, could you have a look please?

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