1179453 - "scroll-snap-type: mandatory" momentum can be interrupted, causing elements to land at incorrect scroll offsets

Status: RESOLVED DUPLICATE of bug 1190469

(Core :: Panning and Zooming, defect)

Description

[Marcus Cavanaugh [:mcav]]

Attachment: Simple HTML test case

When using "scroll-snap-type: mandatory", it is possible/easy to interrupt the scrolling momentum, which causes the element to land on a non-scroll-snapped offset.

This can be demonstrated using the attached simple HTML test case on a touch device, and is also seen in the FxOS task manager (which uses scroll-snap to swipe between open apps).

STR:

1. Swipe (and lift your finger) to cause scroll-snap momentum to kick in.
2. Before the momentum stops, tap quickly anywhere in the scroll-snap element. (i.e. tap, without moving your finger, as though you are interrupting the momentum).
3. Observe that with the momentum stopped, the element does not attempt to scroll to a mandatory scroll offset, and instead rests in between snapping points.

If the steps above are unclear, I can provide video and/or more detailed STR.

Comment 1

[:kip (Kearwood Gilbert)]

(In reply to Marcus Cavanaugh [:mcav] (Mozilla SF) from comment #0)
> Created attachment 8628413 [details]
> Simple HTML test case
> 
> When using "scroll-snap-type: mandatory", it is possible/easy to interrupt
> the scrolling momentum, which causes the element to land on a
> non-scroll-snapped offset.
> 
> This can be demonstrated using the attached simple HTML test case on a touch
> device, and is also seen in the FxOS task manager (which uses scroll-snap to
> swipe between open apps).
> 
> STR:
> 
> 1. Swipe (and lift your finger) to cause scroll-snap momentum to kick in.
> 2. Before the momentum stops, tap quickly anywhere in the scroll-snap
> element. (i.e. tap, without moving your finger, as though you are
> interrupting the momentum).
> 3. Observe that with the momentum stopped, the element does not attempt to
> scroll to a mandatory scroll offset, and instead rests in between snapping
> points.
> 
> If the steps above are unclear, I can provide video and/or more detailed STR.
Thanks for spotting this Marcus.

The intended behavior is that the scroll snapping can be interrupted by a touch gesture; however, releasing the touch gesture should resume the scroll snapping and never leave the scroll position in-between snap points.

Comment 2

[:kip (Kearwood Gilbert)]

I have reproduced this.  It appears that we aren't re-establishing the scroll snapping after a tap (only after flings).  I can take this one.

Comment 3

[Marcus Cavanaugh [:mcav]]

[Blocking Requested - why for this release]:

This shows up very clearly in the new gaia homescreen (bug 1211730).

Comment 4

[Andrew Overholt [:overholt]]

kip, any chance for getting this done in 44?

Comment 5

[:kip (Kearwood Gilbert)]

(In reply to Andrew Overholt [:overholt] from comment #4)
> kip, any chance for getting this done in 44?

I am not likely to have a fix ready in time for 44 as I am currently on the critical path for my current work with WebVR shipping in 44.  After that, I will gladly revisit this.

Comment 6

[Botond Ballo [:botond]]

Bug 1190469 might fix this.

Comment 7

[Kartikaya Gupta (email:[e-mail])]

Indeed, this is basically the same issue as that. I'll take this since I'm looking into that as well.

Comment 8

[Kartikaya Gupta (email:[e-mail])]

The patches I just posted to bug 1190469 fix this issue as well.