Bug 213842 – Missing tiles when tapping back arrow

• Component: Compositing
• Status: ASSIGNED

[Philipp Metzler]

Created attachment 403288 [details]
Animated GIF to illustrate the problem.

Steps to reproduce:

1. On Mobile Safari on iOS 13.5.1 open the URL https://bugreport2.zemtu.com
2. Touch scroll to the red rectangle and click on it
3. Press the arrow / back button
4. Some of the green rectangles on the top of the screen are no longer visible
5. Touch scroll just a little bit to make these rectangles visible again

Please retry several times as it may not occur all the time. Usually you have to try 2 - 4 times to see the problem.

This problem can also be reproduced in the XCode iOS simulator.

The problem cannot be reproduced under macOS Catalina 10.15.5 (19F101) in these browsers:
- Safari Version 13.1.1 (15609.2.9.1.2)
- Google Chrome Version 83.0.4103.116 (Offizieller Build) (64-Bit)
- Firefox 77.0.1 (64-Bit)
- Opera 68.0.3618.173
- Microsoft Edge 83.0.478.56 (Offizielles Build) (64-Bit)

[Radar WebKit Bug Importer]

<rdar://problem/64994694>

[Simon Fraser (smfr)]

I can reproduce on iOS 13.4.

[Simon Fraser (smfr)]

Also reproduces in iOS 14 beta. Sometimes it fails to restore the scroll position.

[Simon Fraser (smfr)]

The failure to restore the scroll position (which also happens on macOS) happens because we try to restore the scroll position from FrameLoader::HistoryController::restoreScrollPositionAndViewState() but this happens before we've done a layout for the new size, so we end up clamping the scroll position to zero.

[Simon Fraser (smfr)]

The page doesn't build the new content until some time after the back(), using JS running on a timer, so we have no hope of restoring the correct scroll position.

[Simon Fraser (smfr)]

On iOS this goes through WebFrameLoaderClient::restoreViewState().