2014726 - [macOS] Split View background area is cut off in vertical mode, collapsed state, when tabs overflow

Status: NEW

(Firefox :: Tabbed Browser: Split View, defect, P3)

Description

[Alexandru Trif, Desktop Test Engineering [:atrif]]

Attachment: unnamed.png

Found in

• 149.0a1 (2026-02-04)

Affected versions

• 149.0a1 (2025-02-04)

Tested platforms

• Affected platforms: macOS 26
• Unaffected platforms: Ubuntu 24, Windows 11

Steps to reproduce

1. In vertical mode, collapsed state, open multiple tabs until they overflow.
2. Create a Split view and observe it

Expected result

• The Split View is displayed as expected.

Actual result

• The Split View background area is cut off.

Regression range

• Happens after bug 1991763.

Additional notes

• Attached a screenshot.

Comment 1

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

Set release status flags based on info from the regressing bug 1991763

:sclements, since you are the author of the regressor, bug 1991763, could you take a look? Also, could you set the severity field?

For more information, please visit BugBot documentation.

Comment 2

[Sarah Clements [:sclements] | PTO until March 30th]

I'm not seeing this, can you confirm this is still an issue?

Comment 3

[Alexandru Trif, Desktop Test Engineering [:atrif]]

Attachment: Screen Recording 2026-02-13 at 2.32.02 pm.mov

(In reply to Sarah Clements [:sclements] from comment #2)

I'm not seeing this, can you confirm this is still an issue?

Hello! Yes, this is still happening with 149.0a1 (2026-02-12) on macOS 26 on a new profile by opening tabs until they overflow and selecting a Split View in Vertical tabs -> collapsed state. If more information is needed, please let me know.