1960053 - scroll-snap-align: start may lead to erratic scrolling in some cases

Status: UNCONFIRMED

(Core :: Layout: Scrolling and Overflow, defect, P2)

Description

[nayinain]

Attachment: testcase.html

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:139.0) Gecko/20100101 Firefox/139.0

Steps to reproduce:

1. Open the testcase.html.
2. Refresh the page with Ctrl + F5.
3. Use the mouse wheel to scroll down several lines in the list, e.g., scroll to the 7th item.
4. Drag the scrollbar to continue scrolling down several lines.
5. Click the list.
6. If unable to reproduce, repeat steps 2-5.

Actual results:

The list will eventually snap back to the 7th item.

Expected results:

The list should not snap back unexpectedly.

---

Sorry for my bad English.

Comment 1

[nayinain]

Attachment: Capture.webm

Comment 2

[nayinain]

The last know good build is 2023-10-30,
The first know bad build is 2023-10-31.

I used mozregression-gui to regression bisection, but could only narrow the issue down to 2023-10-31 due to Unable to find enough data to bisect.

Comment 3

[nayinain]

Attachment: simplifyTestCase2.html

Steps to reproduce:

1. Open the simplifyTestCase2.html.
2. Refresh the page with Ctrl + F5.
3. Use the mouse wheel to scroll down several lines in the list, e.g., scroll to the 7th item.
4. Click the "Scroll down" button.
5. Click the list.
6. If unable to reproduce, repeat steps 2-5.

Comment 4

[nayinain]

Attachment: 2.webm

Comment 5

[nayinain]

Could the following bugs be related to regression?
https://bugzilla.mozilla.org/show_bug.cgi?id=1827975
https://bugzilla.mozilla.org/show_bug.cgi?id=1831580

Comment 6

[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 7

[Botond Ballo [:botond]]

Thanks for filing, and thank you for finding a regression window, this is very helpful!

(In reply to nayinain from comment #2)

The last know good build is 2023-10-30,
The first know bad build is 2023-10-31.

For reference, the commit range corresponding to these dates is https://hg-edge.mozilla.org/mozilla-central/pushloghtml?fromchange=9c6272da5dc92ff944d294d9a026bd42a408c784&tochange=2c44d59dd29eb01ea0b484c9f3f8691861118073.

You are right that in that range, bug 1827975 and bug 1831580 are the ones which seem related to scrolling.

Using a local backout, I confirmed that bug 1831580 is the regressing change.

Comment 8

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

:nayinain, since this bug is a regression, could you fill (if possible) the regressed_by field?
For more information, please visit BugBot documentation.

Comment 9

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

:dlrobertson, since you are the author of the regressor, bug 1831580, could you take a look?

For more information, please visit BugBot documentation.

Comment 10

[Dan Robertson (:dlrobertson)]

Started investigating this and found the specific change that causes this... Still figuring out a way to fix this in a way that doesn't regress bug 1831580