2009567 - Group line of a tab may disappear during dragging when the previous item is not a tab

Status: NEW

(Firefox :: Tabbed Browser: Tab Groups, defect, P3)

Description

[Merci chao]

Attachment: 2026_01_10_20_34_41.mp4

1. Drag a tab inside a group where its previous item is not a tab.
2. Move backward a little to overlap the previous item, without moving over it.
3. Move forward slightly, the group line of the dragged tab disappears.

Comment 1

[Ardelean Oana, Desktop Test Engineering [:oardelean]]

Hello, thank you for the bug report!
I managed to reproduce this issue:

• Firefox Nightly 149.0a1;
• Firefox 148.0b1;
• Firefox 147.0;

Tested and reproduced on:

• Windows 10;
• Ubuntu 22;
• macOS 15;

Setting as NEW so the developing team can have a look.

Comment 2

[Stephen Thompson [:sthompson]]

Thank you for the report! I think we may have a bug on file that covers a similar behavior -- when you drag an ungrouped tab near the last tab of a group, the dragged tab's group line will disappear very quickly if you move just a pixel toward the end of the tab group. There may be a shared cause. I know that the drag-drop code in general is sensitive about whether one is dragging "forward" or "backward."