1975159 - Tab scrollbar has emtpy space after moving a tabgroup to another window

Status: NEW

(Firefox :: Tabbed Browser, defect)

Description

[Lucas]

Attachment: Bildschirmaufnahme 2025-07-02 um 11.41.54.mov

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:140.0) Gecko/20100101 Firefox/140.0

Steps to reproduce:

• Create anough tabs that the tabs start scrolling horizontally.
• Create a tab group. There still have to be enough tabs to still scroll.
• Open the tabgroup
• Drag the tab group to another window.

If the tabgroup is closed while dragging it to the other window the issue doesn't happen.

Actual results:

The tab scrollbar now has a empty space at the end. This persists until the window is closed.

Expected results:

It shouldn't scroll into the empty space / be resized to fit the tabs that are actually there.

Comment 1

[Lucas]

I don't know why it says Intel Mac in the User Agent string. I have an arm mac and when opening the "About Firefox" window it says 140.0.2 (aarch64) as the version.

Comment 2

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

The Bugbug bot thinks this bug should belong to the 'Firefox::Tabbed Browser' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Comment 3

[Negritas Sergiu, Desktop QA]

Hello! I have managed to reproduce the issue with firefox 142.0a1(2025-07-03) on MacOS and Ubuntu 22.04.

I will mark this issue as NEW in order to get our developers involved and provide a solution for it in future releases.

Have a nice day!