Bug 216926

Summary: Scroll container reverts back to previous snap point while the user is scrolling
Product: WebKit Reporter: Johannes Odland <johannes.odland>
Component: ScrollingAssignee: Nobody <webkit-unassigned>
Status: RESOLVED FIXED    
Severity: Normal CC: johannes.odland, simon.fraser, webkit-bug-importer
Priority: P2 Keywords: InRadar
Version: Safari 14   
Hardware: iPhone / iPad   
OS: iOS 13   
Attachments:
Description Flags
Video of the issue on an iPhone 11 Pro with iOS 14 none

Description Johannes Odland 2020-09-24 04:59:47 PDT 
Created attachment 409559 [details]
Video of the issue on an iPhone 11 Pro with iOS 14

Some users experience that the scroll container reverts to the previous snap point while they are scrolling. 
The issue might be caused by reflow. 

I've managed to reproduce the bug with a small example: https://codepen.io/johannesodland/pen/xxVmamJ

We've tested this on two similar iPhone 11 Pros on iOS 14. 
One of the phones snaps back to the original snap point while the user is scrolling. This does not happen on the other phone.
Comment 1 Radar WebKit Bug Importer 2020-09-27 10:05:27 PDT 
<rdar://problem/69660064>
Comment 2 Johannes Odland 2020-09-27 23:02:21 PDT 
This issue breaks websites with scroll snap on some devices.
Comment 3 Simon Fraser (smfr) 2020-09-28 15:28:11 PDT 
Do you know what's different about the two phones? Are they both running the exact same build?
Comment 4 Simon Fraser (smfr) 2020-09-28 15:32:14 PDT 
I can reproduce the bug you describe in iOS 13.x, but not in iOS 14.
Comment 5 Johannes Odland 2020-09-29 01:16:00 PDT 
The phone with the issue runs build 18A5332f
Comment 6 Simon Fraser (smfr) 2020-09-29 09:27:48 PDT 
And what OS is the phone without the issue running?
Comment 7 Johannes Odland 2020-09-29 10:28:38 PDT 
The phone without the issue has been updated since reporting the issue. It was iOS 14, it is now iOS 14.0.1 18A393
Comment 8 Simon Fraser (smfr) 2020-09-29 11:23:27 PDT 
So I think it's to say that the issue has been fixed between 18A5332f and 18A393.
Comment 9 Johannes Odland 2020-10-02 01:29:05 PDT 
The bug is still there in 14.0.1 18A5332f.
Comment 10 Johannes Odland 2020-10-02 03:02:49 PDT 
I'm sorry. The phone was still running the old build. 
Can confirm that the issue is fixed in 14.0.1. 18A393.