Can't drag down on iOS 13

[fokayx]

Describe the bug
While iOS upgrade to 13, our dnd sortable list component can drag up but can't drag down. All it worked in iOS < 12. Is it a bug in iOS 13 ? Have any idea?

Desktop (please complete the following information):

• OS: iOS
• Browser: chrome, safari
• Version 78.0.3904.67

[fsp]

I've also observed this problem starting iOS 13.1 and it is still present in iOS 13.2 beta. I am using a custom drag layer with the touch backend.

This seems to be an issue with WebKit on iOS specifically.

[zhusee2]

Also experiencing the same thing on iOS 13.2 release. It seems to be happening in the “sorting” context.

While you can drag a node anywhere trouble-free, if you re-render a list when you drag item A downward onto item B, you can suddenly loose control to the dragging node. But the drag isn't considered as end–the endDrag() callback is triggered only when your finger really leaves the screen.

It almost happen every time as you drag downward and and re-render. It can also happen when you drag upward, but that's a relatively rare case.

[zhusee2]

I took a closer look at the touch backend, and I found it's stuck at TouchBackend.ts#L390 because it can't find event client offset.

Turned out that in iOS 13, event.targetTouches list from touchmove event can be cleared out in the above situation, even when the only Touch target in its event.touches is the same node from touchstart event.

[fsp]

This may be related to this webkit issue

[varseb]

Same issue here.
Still present on last IOS update to 13.2.2

[thenano]

I'm also experiencing this and have confirmed that everything works as expected on iOS 12.
Thanks for providing a fix @zhusee2 !
Hopefully the fix can be merged soon. Thanks all