Bug 236312 - Rapid changes in scroll position can cause JS-exposed scroll offsets to be stale
Summary: Rapid changes in scroll position can cause JS-exposed scroll offsets to be stale
Status: NEW
Alias: None
Product: WebKit
Classification: Unclassified
Component: Scrolling (show other bugs)
Version: Safari 15
Hardware: iPhone / iPad Unspecified
: P2 Major
Assignee: Nobody
URL:
Keywords: InRadar
Depends on:
Blocks:
 
Reported: 2022-02-08 10:18 PST by Jack Doyle
Modified: 2023-02-23 05:50 PST (History)
3 users (show)

See Also:

Attachments
Testcase (2.21 KB, text/html)
2022-02-15 14:44 PST, Sam Sneddon [:gsnedders] 		no flags Details
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Description Jack Doyle 2022-02-08 10:18:41 PST 
Set the scroll position via JavaScript and then immediately check it and it seems to work correctly but under heavy load it actually REVERTS to a previous scroll position on the next requestAnimationFrame()! From what I can tell, it's definitely a bug. Here is the proof (you must run this on iOS Safari in the /debug/ window of CodePen so that it's not in an iframe): https://codepen.io/GreenSock/pen/16c435b12ef09c38125204818e7b45fc?editors=0010

Again, this only happens under heavy load. 

I understand JS runs on a separate thread from scrolling but all of this is happening in JS world so the regression of the scroll position doesn't make sense to me. 

Can anyone explain why this is happening and how we can work around it?
Comment 1 Radar WebKit Bug Importer 2022-02-14 19:54:25 PST 
<rdar://problem/88943486>
Comment 2 Sam Sneddon [:gsnedders] 2022-02-15 14:44:11 PST 
Created attachment 452091 [details]
Testcase

I couldn't get the codepen to load in debug mode, so here's it attached.

<rdar://88342742>
Comment 3 Simon Fraser (smfr) 2022-02-15 15:22:00 PST 
Scrolling on iOS involves an asynchronous round-trip to a different process, and it's possible we're getting a stale scroll position back to JS because of that.

I don't know of any workarounds currently.
Comment 4 Cassie Evans 2023-02-23 05:50:51 PST 
Are there any intentions to look into this?