1899380 - Scroll position snaps back to start, with `scroll-snap` and fractional margin

Status: NEW

(Core :: Layout: Scrolling and Overflow, defect, P3)

Description

[Daniel Holbert [:dholbert]]

Attachment: testcase 1

[Spinning this off from bug 1850485, for an issue that I identified while poking at that. I'm not entirely sure that this is the full explanation there, but it might be.]

STR:

1. Load attached testcase.
2. Click the button a few times to increase the scroll position. Or, simply click and drag the scrollbar (in the scrollable box) all the way to the right.

EXPECTED RESULTS:
The scrollable box should be able to reach to the maximum (rightmost) position, and you should see the text "SUCCESS" when it reaches that position.

ACTUAL RESULTS:

• The scrollbar resets back to the start (leftmost) position, when you would expect it to reach the right edge.
• You can scroll to intermediate positions, but when it reaches the right edge (or potentially surpasses it via clicking the button), it unexpectedly resets (snaps back to the start).

Comment 1

[Daniel Holbert [:dholbert]]

Attachment: reference case

Here's a reference case, where I simply changed margin-inline-start from 10.5 to 11px.

This is sufficient to make the testcase give EXPECTED RESULTS.

Comment 2

[Daniel Holbert [:dholbert]]

Attachment: testcase 2 (no interaction required)

Comment 3

[Daniel Holbert [:dholbert]]

Regression range:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=75b3c6c4437624f59fcaff278bb8f5b88358e3a6&tochange=f476897a6e6ac50ecebdf7f8bda821d40aab066c

Suspect: bug 1817126

hiro, could you take a look here?

Comment 4

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

Set release status flags based on info from the regressing bug 1817126

:hiro, since you are the author of the regressor, bug 1817126, could you take a look? Also, could you set the severity field?

For more information, please visit BugBot documentation.

Comment 5

[Daniel Holbert [:dholbert]]

Note: If I set layout.scroll.disable-pixel-alignment to true, then I can't repro anymore.

Comment 6

[Hiroyuki Ikezoe (:hiro)]

Thank you, Daniel! This will be fixed by bug 1852884.

Comment 7

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

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

For more information, please visit BugBot documentation.

Comment 8

[Hiroyuki Ikezoe (:hiro)]

We are going to tackle bug 1852884 and relevant ones in this half.