1975145 - Tabs jiggle when starting drag with ui.prefersReducedMotion=1

Status: RESOLVED FIXED

(Firefox :: Tabbed Browser, defect, P1)

Description

[6k64x4ma]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:141.0) Gecko/20100101 Firefox/141.0

Steps to reproduce:

1. Set ui.prefersReducedMotion to 1 in about:config.
2. Use Horizontal tabs.
3. Add enough tabs to fill the tabstrip, but not so many that scrollbuttons become visible.
4. Drag an unpinned tab.

Actual results:

When I start dragging a tab, the other tabs jiggle.

Expected results:

The tabs should not jiggle.

Regression range:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=559d435750e29dbe7b4a4c8297f4222b596b14d5&tochange=fdd655561d6df0b7b96450a0aa34eb7e964e2f0a

Comment 1

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

:nsharpley, since you are the author of the regressor, bug 1944907, could you take a look? Also, could you set the severity field?

For more information, please visit BugBot documentation.

Comment 2

[Sarah Clements [:sclements]]

I'm not seeing this - could you share a screencast please?

Comment 3

[6k64x4ma]

Attachment: firefox_eZAHMvcmtc.mp4

Comment 4

[Stephen Thompson [:sthompson]]

Can't reproduce on macOS, I'll try on Windows.

Comment 5

[Stephen Thompson [:sthompson]]

Confirmed on Windows 11 with Nightly 142.0a1 (2025-07-02). Based on the behavior, I'm guessing this is indeed regressed by bug 1944907.

Comment 6

[Alice0775 White]

I can reproduce the issue on Windows11 as well as Ubuntu22.04.

Comment 7

[Pascal Chevrel:pascalc]

The feature that caused this regression was backed out of beta in bug 1975426, marking 141 as unaffected

Comment 8

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

The bug is marked as tracked for firefox142 (nightly). We have limited time to fix this, the soft freeze is in 8 days. However, the bug still isn't assigned and has low severity.

:cbellini, could you please find an assignee and increase the severity for this tracked bug? Given that it is a regression and we know the cause, we could also simply backout the regressor. If you disagree with the tracking decision, please talk with the release managers.

For more information, please visit BugBot documentation.

Comment 9

[Stuart Colville [:muffinresearch]]

Assigning this to Sthompson to take a look alongside bug 1972897 since they're likely to be closely related.

Comment 10

[Stephen Thompson [:sthompson]]

Ubuntu 24.04 on X11, this is occurring in the horizontal tab strip only.

Unlike bug 1972897, these issues occur with both grouped tabs and ungrouped tabs.

Just like bug 1972897, these issues are resolved by replacing the fixed-max-width code in #updateTabStylesOnDrag with _lockTabSizing() and calling _unlockTabSizing() in #resetTabsAfterDrop.

I'll open a patch with this proof-of-concept fix in order to get feedback about potential negative side effects.

Comment 11

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

Set release status flags based on info from the regressing bug 1944907

Comment 12

[Dianna Smith [:diannaS]]

Marking as fixed by bug 1972897