1969164 - When a tab group is in the only thing in a window, dragging it to another window leaves an empty window

Status: VERIFIED FIXED

(Firefox :: Tabbed Browser, defect, P3)

Description

[Stephen Thompson [:sthompson]]

Attachment: A screenshot showing that the new window remains with an empty tab strip after dragging the "bridges" tab group from the new window back to the original window.

Nightly 141.0a1 (2025-05-28) (aarch64)
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:141.0) Gecko/20100101 Firefox/141.0

Steps to reproduce:

• Open two tabs, A and B
• Create a tab group for tab B
• Drag the tab group label outside of the browser window and drop it -- this should trigger the behavior to create a new window containing only the dragged tab group
• In the new window, drag the tab group label and drop it back into the original window

Actual results:

The tab group correctly moves back to the original window. The new window remains in place, but there is nothing in the tab strip. The URL bar contains the URL of the previously active tab, but the browser panel has no content.

Expected results:

The tab group correctly moves back to the original window. The new window automatically closes because there is no longer anything in that window.

Comment 1

[Stephen Thompson [:sthompson]]

I can reproduce this in the following scenarios:

• tab group was moved to a new window using drag-drop
• tab group was created manually in a new window using the tab context menu

The new window automatically closes as expected in the following scenarios:

• tab group was moved to new window from tab group context menu
• tab group was moved to new window from list all tabs menu context menu
• tab group was created in a new window in a previous session that was restored manually

Not sure exactly what to make of it. It seems like the aCloseWindow from _endRemoveArgs is false for the last tab to be adopted when things go wrong, whereas that value is true when things go correctly.

I haven't dug much further. This logic in Tabbrowser caught my eye https://searchfox.org/mozilla-central/rev/932e5cded30452c4f009b4f9254021f8b9895217/browser/components/tabbrowser/content/tabbrowser.js#4829-4833 and I wondered if perhaps the collapsed-during-drag state of the tab group could interfere with window closing behavior, but I don't see why that behavior would vary based on the different scenarios I listed above.

Comment 2

[Dão Gottwald [:dao]]

This is just a guess, but could this be a regression from bug 966594?

Comment 3

[Stephen Thompson [:sthompson]]

Aha, I didn't notice that the code I linked had just changed in bug 1966594. Yeah, I'll test that, but it looks this is a regression.

Comment 4

[Stephen Thompson [:sthompson]]

I can reproduce with the original code in place

Comment 5

[Stephen Thompson [:sthompson]]

Attachment: Bug 1969164 - close window when dragging tab group out of window r=dwalker,jswinarton

When a window consists only of a tab group and its tabs, dragging the tab group into the tab strip of another window can leave the starting window in a broken, empty state.

The normal behavior when dragging a tab group is to temporarily collapse the tab group, but this seems to have an unintended interaction with the Tabbrowser logic that decides whether to close a window when the last tab is removed. At the time of the last tab being removed, it is in a temporarily collapsed group and therefore not visible.

This patch hardens the logic for calculating whether a tab is the last tab in a window. It ensures that the window will close if the last tab in the window is in a collapsed tab group.

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/bec968241787
https://hg.mozilla.org/integration/autoland/rev/367bc904b4fd
close window when dragging tab group out of window r=jswinarton,tabbrowser-reviewers,dao

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/9318e6027de8
https://hg.mozilla.org/integration/autoland/rev/740dab811f0b
Revert "Bug 1969164 - close window when dragging tab group out of window r=jswinarton,tabbrowser-reviewers,dao" for causing failures complaining about tabs.

Comment 8

[Atila Butkovits]

Backed out for causing failures complaining about tabs.

Backout link: https://hg-edge.mozilla.org/integration/autoland/rev/740dab811f0b8563cbb9a5551dafca20b4cf9531

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=367bc904b4fd00b55b628badd4a50a63bf578a9a&selectedTaskRun=DMH3-cG3Qze_crP8GC3ogQ.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=511960161&repo=autoland&lineNumber=1481

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/cd0c95e428c4
https://hg.mozilla.org/integration/autoland/rev/0f12899d99ae
close window when dragging tab group out of window r=jswinarton,tabbrowser-reviewers,dao

Comment 10

[Narcis Beleuzu [:NarcisB]]

https://hg.mozilla.org/mozilla-central/rev/0f12899d99ae

Comment 11

[Dana Lucaci, Desktop QA]

Reproduced with Firefox 141.0a1 (2025-05-28) on macOS 15.
Verified as fixed with Firefox 142.0a1 (20250626214351) and Firefox 141.0b3 (20250627042037) on macOS 14, Windows 10 and Ubuntu 22.