1893306 - Unexpected horizontal scrolling at the end of a vertical pan - lifting the finger breaks the axis lock

Status: NEW

(Core :: Panning and Zooming, defect, P3)

Description

[Markus Stange [:mstange]]

Attachment: screen recording

On Android, when panning to the bottom of a page that's scrollable on both axes, the page sometimes scrolls horizontally once the finger is lifted.

Steps to reproduce:

1. Go to https://phabricator.services.mozilla.com/D208427
2. Scroll to the bottom with a series of flings.

Sometimes the page ends up being scrolled horizontally once you reach the bottom, because the scroll gesture happened to be slightly diagonal.

This is unexpected - if we only scroll vertically while the fingers are on the screen, then we should continue to only scroll vertically once the fingers are no longer on the screen.

---

I think what happens is that, when we initiate the fling at the end of the pan, we don't respect the current axis lock, so we can initiate a horizontal fling even if the pan was locked vertically.

Comment 1

[Markus Stange [:mstange]]

Attachment: easier to reproduce screen recording

It's often easier to reproduce this in the other direction - horizontal pans starting vertical flings.

In this recording, I did a horizontal pan, and then started moving my fingers vertically, before I lifted them. The vertical motion is ignored while the fingers are on the screen, but then gets translated into a vertical fling once the fingers are lifted.

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

The severity field is not set for this bug.
:botond, could you have a look please?

For more information, please visit BugBot documentation.

Comment 3

[Dan Robertson (:dlrobertson)]

Attachment: Bug 1893306 - Fling animations should respect the gesture axis lock. r=botond

Ensure that a fling animation following a touch scroll respects the axis
lock used durring the gesture.

Comment 4

[Frank Griffith Jr]

Are we waiting for additional changes or reviews on these patches? Just checking in. Thanks.

Comment 5

[Dan Robertson (:dlrobertson)]

Just got some extra info on the patch... I'll figure out next steps today.

Comment 6

[Dan Robertson (:dlrobertson)]

Attachment: Bug 1893306 - Do not let all attempted flings handoff. r=botond

Not all flings should allow for handoff. A overscroll fling should be
able to handoff residual velocity to a APZC earlier in the overscroll
handoff chain, but a fling that starts in an APZC that is not scrollable
in the given direction should not handoff.