2043753 - [APZ] Scrolling stutter strictly triggered on initial touch-move / panning setup when interrupting or restarting a fling

Status: RESOLVED FIXED

(Core :: Panning and Zooming, defect)

Description

[Skywarp]

User Agent: Mozilla/5.0 (Android 16; Mobile; rv:153.0) Gecko/153.0 Firefox/153.0

Steps to reproduce:

1. Open about:config (I find it best reproducible there)
2. Try to scroll fast through the page

Actual results:

It is very stuttery, kind of hard to explain

Expected results:

It should not lag like this

Comment 1

[Skywarp]

Video attachment

Comment 2

[Skywarp]

It also happens on a completely new profile. Reproduced on SM-S911B

Comment 3

[Skywarp]

It happens on some (most) websites, but not on every each of them, weirdly. I have requested a needinfo because this is a serious performance regression that heavily impacts usability on recent Nightly builds. Not sure if it will help.

Comment 4

[Skywarp]

I have successfully run a full bisection using mozregression on the GeckoView Example app (gve), which confirms that this is a Core Gecko engine regression rather than a Fenix UI issue.

The scrolling stutter reproduces perfectly inside GVE, and it isolates exactly to the merge of Bug 2041154.

Bisection Results:

• Last Known Good Revision: 914f2a503c1d346dae82b4ed9f270b1f39fa29a6
• First Known Bad Revision: 909577e462d4c5e34edd058fb59b26c42b17f534
• Pushlog URL: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=914f2a503c1d346dae82b4ed9f270b1f39fa29a6&tochange=909577e462d4c5e34edd058fb59b26c42b17f534

The massive stutter happens right at the initial touch gesture when starting to pan/scroll. This matches the behavior introduced in changeset 909577e462d4 ("Wait for a content response on the first touch-move if needed"), where APZ is now blocking on the main thread during the initial input event.

Moving this bug to Core :: Pan and Zoom so the right team can investigate or look into a backout.

Comment 5

[Skywarp]

Note: The stutter only occurs on the initial input action (touch-move / panning setup). If a fast scroll is initiated and left to decelerate naturally via momentum (fling) without any further touch inputs, the scrolling remains perfectly smooth. The jank strictly triggers the exact moment the finger touches the screen again to interrupt or restart panning, which perfectly points to APZ blocking while waiting for a main-thread content response on the first touch-move event.

Comment 6

[Skywarp]

I'll admit the needinfo was useless, I'm clearing it. Sorry for this.

Comment 7

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

:hiro, since you are the author of the regressor, bug 2041154, could you take a look? Also, could you set the severity field?

For more information, please visit BugBot documentation.

Comment 8

[Hiroyuki Ikezoe (:hiro)]

Bug 2041154 was backed out.