1962688 - Dragging tab a small amount detaches it into a new window when the parent window is not maximized

Status: VERIFIED FIXED

(Firefox :: Tabbed Browser, defect, P1)

Description

[Kestrel]

Attachment: Tab detaches with small drag movement

STR:

1. Unmaximize the window on Ubuntu 25.04.
2. Make sure there are at least two tabs.
3. Drag a tab sideways a small amount and release just before it starts moving without leaving tab strip.
4. Repeat a few times.

Expected:
Tab remains in tab strip.

Actual:
Tab detaches into a new window.

Does not happen for maximized windows.

Still happens with:
browser.tabs.hoverPreview.enabled = false
nglayout.enable_drag_images = false

Easy to reproduce with Wayland but more difficult with XWayland.

Workaround is to change browser.tabs.allowTabDetach = false.

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=ff5432cc7e7c0d223330a7c8f09a89d797793f83&tochange=9efb94e533503b0cd1309c16d01d488a69941059

Regressed by Bug 581863.

Comment 1

[BugBot [:suhaib / :marco]]

:emilio, since you are the author of the regressor, bug 581863, could you take a look? Also, could you set the severity field?

For more information, please visit BugBot documentation.

Comment 2

[Emilio Cobos Álvarez [:emilio]]

I think I can repro this... I'm not super familiar with our tab dragging code to know off the top of my head, but I looked at this and I think the event coordinates look off.

I think this code is now wrong. The coordinates of the event are supposed to be relative to the client area.

Comment 3

[Emilio Cobos Álvarez [:emilio]]

Hmmm actually I was wrong... I think the code is correct, it's just the front-end which is wrong.

Comment 4

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1962688 - Fix tab drop zone calculation to account for window decorations correctly. r=#tabbrowser-reviewers

This is more visible on Linux because the decorations there are bigger, but I
believe this also fixes issues on windows specially if you have the titlebar
checkbox on.

You need to use mozInnerScreen* coords in order to get the screen coordinates
of the client area.

Comment 5

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1962688 - Fix vertical tabs on right drop zone calculation. r=#tabbrowser-reviewers

Noticed this while going through this code. The end should be all the
way to the right, not left...

Comment 6

[BugBot [:suhaib / :marco]]

The product::component has been changed since the backlog priority was decided, so we're resetting it.
For more information, please visit BugBot documentation.

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/ea19d60a97e9
Fix tab drop zone calculation to account for window decorations correctly. r=tabbrowser-reviewers,nsharpley
https://hg.mozilla.org/integration/autoland/rev/5eaadf8976dc
Fix vertical tabs on right drop zone calculation. r=tabbrowser-reviewers,nsharpley

Comment 8

[Serban Stanca [:SerbanS]]

https://hg.mozilla.org/mozilla-central/rev/ea19d60a97e9
https://hg.mozilla.org/mozilla-central/rev/5eaadf8976dc

Comment 9

[Rares Doghi, Desktop QA]

Verified as fixed in our latest Nightly 140.0a1 (2025-04-29)

Comment 10

[Dão Gottwald [:dao]]

Thanks Rares! Let's get this uplifted to Beta?

Comment 11

[Emilio Cobos Álvarez [:emilio]]

Comment on attachment 9481325 [details]
Bug 1962688 - Fix tab drop zone calculation to account for window decorations correctly. r=#tabbrowser-reviewers

Beta/Release Uplift Approval Request

• User impact if declined/Reason for urgency: comment 0
• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: Yes
• If yes, steps to reproduce: comment 0
• List of other uplifts needed: none
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Simple coordinate space fixes.
• String changes made/needed: none
• Is Android affected?: No

Comment 12

[Ryan VanderMeulen [:RyanVM]]

Comment on attachment 9481325 [details]
Bug 1962688 - Fix tab drop zone calculation to account for window decorations correctly. r=#tabbrowser-reviewers

Approved for 139.0b2.

Comment 13

[Pulsebot]

https://hg.mozilla.org/releases/mozilla-beta/rev/18413fdad24d
https://hg.mozilla.org/releases/mozilla-beta/rev/0fe393b538d3

Comment 14

[Rares Doghi, Desktop QA]

This issue is verified as fixed in our latest Beta 139.0b2 20250430022719