Bug 191218 – Touch event not firing while scrolling is in motion

• Component: UI Events
• Status: NEW

[nick]

When user scrolls and in the deceleration phase of the inertial scrolling, user touch the screen will make the scrolling stop but no "touch" event is fired. I'd like to detect where the user touch to stop the scrolling but I can't because there's no event firing. Safari and Chrome on iphone have this issue while Android doesn't.
I've created a CodePen for testing.
https://codepen.io/anon/pen/MzWKrM

Please help to fix this.

[Frédéric Wang (:fredw)]

Created attachment 384303 [details]
testcase

Reproducible in iOS 13.3 17C5046a

[Frédéric Wang (:fredw)]

Preliminary debugging on the simulator (public SDK):

- When you scroll and tap the blue rectangle during scrolling, WKScrollingNodeScrollViewDelegate::scrollViewDidEndDragging is called and just informs ScrollingTreeScrollingNodeDelegateIOS about the scroll & scroll end.

- When you tap directly the blue rectangle, WKContentView::_singleTapGestureRecognizer will invoke WebPageProxy::commitPotentialTap in order to tell the Web process to dispatch an event.

- WKContentViewInteraction::gestureRecognizerShouldBegin has some logic when tap interrupts a scroll deceleration, but the code does not seem to be called for me.

I wonder if there is something wrong in the fact that touch gesture are not handled by WKContentView during scrolling or if the scrolling delegate should do similar work as WKContentView to recognize touch gestures.

[Frédéric Wang (:fredw)]

(In reply to Frédéric Wang (:fredw) from comment #2)
> - When you scroll and tap the blue rectangle during scrolling,
> WKScrollingNodeScrollViewDelegate::scrollViewDidEndDragging is called and
> just informs ScrollingTreeScrollingNodeDelegateIOS about the scroll & scroll
> end.

I tried sending the commitPotentialTap message from there but it does not work. The target node & position must be set first with a potentialTapAtPosition, so it seems the solution will be a bit more complex.

> - When you tap directly the blue rectangle,
> WKContentView::_singleTapGestureRecognizer will invoke
> WebPageProxy::commitPotentialTap in order to tell the Web process to
> dispatch an event.

WKContentView::_singleTapGestureRecognizer is a WKSyntheticTapGestureRecognizer. However, this listener is not triggered when the UIScrollView is scrolling...

> I wonder if there is something wrong in the fact that touch gesture are not
> handled by WKContentView during scrolling or if the scrolling delegate
> should do similar work as WKContentView to recognize touch gestures.

If the former, I guess that's something Apple should fix. For the latter, a proper solution will probably be to use some UITapGestureRecognizer on UIScrollView, if that's ever possible. I'll investigate more on this but found  https://stackoverflow.com/questions/27715448/uitapgesturerecognizer-on-uiscrollview