Bug 239605 – Focusing scroll container before scrolling breaks smooth scrolling

• Status: RESOLVED
• Resolution: FIXED
• Component: Scrolling

[Liam DeBeasi]

Created attachment 458061 [details]
Code reproduction

Focusing a scroll container before performing a smooth scroll operation causes smooth scrolling to not work. Non-smooth scrolling works as expected.

Steps to reproduce:

1. Open attached code reproduction on an iOS device that supports smooth scrolling.
2. Tap the "Smooth scroll to end" button. Observe that the container does not scroll to the end.
3. Tap the "Non Smooth scroll to end" button. Observe that the container scrolls to the end.

Expected Behavior:

I would expect that the container scrolls regardless of smooth scroll status.

Actual Behavior:

The scroll container does not scroll if smooth scrolling is enabled.

Other Information:

- This works as expected on Chrome and Firefox.
- Disabling CSSOM View Smooth Scrolling in Settings > Safari > Experimental Features resolves the issue.
- Wrapping the scroll operation in a requestAnimationFrame fixes the issue.

[Ryan Haddad]

rdar://92105675

[Simon Fraser (smfr)]

Also reproduces on macOS.

[Simon Fraser (smfr)]

The focus call is triggering a call to stopAsyncAnimatedScroll:

WebPageProxy 6 activityStateDidChange - mayHaveChanged active window
WebPageProxy 6 dispatchActivityStateChange - potentiallyChangedActivityStateFlags active window
WebPageProxy 6 dispatchActivityStateChange: state changed from active window, focused, visible, visible or occluded, in-window to focused, visible, visible or occluded, in-window
WebPage 7 setActivityState to focused, visible, visible or occluded, in-window
RenderLayer 0x59b04d540 584x250 stopAsyncAnimatedScroll

[Simon Fraser (smfr)]

That might be a red herring, or only happen the first time. On every subsequent button click, there's no activity state change, but the animated scroll is getting cancelled because FrameView::scrollToFocusedElementInternal() runs on a timer.

[Simon Fraser (smfr)]

Created attachment 458112 [details]
Patch

[Simon Fraser (smfr)]

Created attachment 458113 [details]
Patch

[Simon Fraser (smfr)]

Created attachment 458155 [details]
Patch

[EWS]

Committed r293260 (249904@main): <https://commits.webkit.org/249904@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 458155 [details].

[Liam DeBeasi]

Thank you for the fix!