[Webkit-unassigned] [Bug 209486] Drifting when scrolling an iframe that scrolls on both axes

Wed Apr 29 07:20:22 PDT 2020

https://bugs.webkit.org/show_bug.cgi?id=209486

--- Comment #14 from cathiechen <cathiechen at igalia.com> ---
(In reply to Dima Voytenko from comment #13)
> A bit more info: I did some more testing with `touch-action: pan-y` in hopes
> that the scroll behavior would improve, but this doesn't seem to affect it.

It seems touch-action is based on ENABLE(TOUCH_EVENTS). Only _handleTouchActionsForTouchEvent in WKContentViewInteraction.mm changes the touchActions values. But TOUCH_EVENTS seems can't be enabled by using the public SDK, see bug 179167 for details.
https://output.jsbin.com/kezoqumeni
The above test added `touch-action: pan-y;` to <html> of the iframe. The horizontal scrolling is locked if open the page on the original safari of iPhone(13.4.5) or iOS-simulator(13.3). Note, there's an issue about touch-action, it seems can't lock the horizontal scroll during momentum scrolling. 

The drifting issue seems related to UIScrollView, from the outside, maybe we can add a constraint to avoid unwanted horizontal/vertical scrolling as touch-action does in _scrollView and scrollViewWillEndDragging? But it seems still can't avoid it if it's during momentum scrolling.

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