1996353 - Tabs overlap and the top bar is unresponsive after switching/moving tabs.

Status: UNCONFIRMED

(Firefox :: Tabbed Browser, defect)

Description

[fokko.perton+bugzilla]

Attachment: Screenshot_20251024_200711.png

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:144.0) Gecko/20100101 Firefox/144.0

Steps to reproduce:

Drag tabs on the top bar around or in rare cases even just switching tabs.

Actual results:

Tab that was dragged stays in the same location, but becomes unclickable. Opening a new tab or dragging another tab causes them to overlap. The navigation bar also becomes entirely unresponsive to the mouse, but still works when accessing with f6.

Expected results:

The tabs change position without overlapping and the navigation bar stays responsive to the mouse.

Comment 1

[fokko.perton+bugzilla]

Attachment: about:support output

Comment 2

[fokko.perton+bugzilla]

As this bug is similar to this bug I tried replicating it with the console open. After a minute or 2 of dragging tabs around I got the bug, but the output of the console doesn´t seem very useful. Here it is nonetheless:
1761329529502 addons.xpi WARN Checking /usr/lib/firefox/distribution/extensions for addons Unexpected non-empty DNRStore data. DNR startupCache data load dropped. ExtensionDNRStore.sys.mjs:1228 WebGL context was lost. 2 Troubleshoot.sys.mjs:726:17 In add-on {446900e4-71c2-419f-a6a7-df9c091e268b}, attempting to use listener "notifications.onButtonClicked", which is unimplemented. ExtensionParent.sys.mjs:1330:13 Key event not available on some keyboard layouts: key=“z” modifiers=“control,alt” id=“toggleSidebarKb” browser.xhtml Key event not available on some keyboard layouts: key=“x” modifiers=“accel,alt” id=“viewGenaiChatSidebarKb” browser.xhtml Key event not available on some keyboard layouts: key=“r” modifiers=“accel,alt” id=“key_toggleReaderMode” browser.xhtml Key event not available on GTK2: key=“u” modifiers=“accel,shift” id=“” browser.xhtml Key event not available on some keyboard layouts: key=“i” modifiers=“accel,alt,shift” id=“key_browserToolbox” browser.xhtml Missing resource in locale en-CA: devtools/client/toolbox.ftl Missing resource in locale en-GB: devtools/client/toolbox.ftl Referrer Policy: Ignoring the less restricted referrer policy “unsafe-url” for the cross-site request: https://www.redditmedia.com/gtm/jail?cb=8CqR7FcToPI jail WebGL context was lost. 2 Troubleshoot.sys.mjs:726:17
I do get this bug fairly often, 2-4 times a day. I have tried refreshing firefox to remedy it, but have not yet tried firefox in safe mode nor a full re-install.

Comment 3

[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 4

[Jeremy Swinarton (PTO until 2026-04-06)]

Hi Rares, would you or someone else from QA be able to try to reproduce this? We thought this was fixed by bug1984199, but it still seems to be happening in some cases. Thank you!

Comment 5

[Rares Doghi, Desktop QA]

Attachment: Screencast from 2025-10-29 14-08-43.webm

Hi @Jeremy, I'm not entirely sure but I think this is Bug 1974475. When I have a pinned tab or a few pinned tabs and a few free tabs, If I throw them around inside the Tab strip especially around that Pinned section, I do get this overlap bug on Ubuntu only, Windows and Mac are not affected by this issue, the problem is I cant reproduce this 100% every time, but if I try enough times I will get this to happen, I was able to reproduce this issue in Release 144, Beta 145.0b7 as well as our latest Nightly build.

I dont know if its my Ubuntu machine or my GPU or a combination of the two but others have tried it with my steps and it doesn't happen. Its also hard to record this issue since my entire OS freezes for a few seconds and after it starts running again the Screen recording gets distorted. I was lucky enough to reproduce it before that happened.

Please also note that the entire tabstrip is frozen, I can only click the Tab Icons and sometimes the X Close and Maximize buttons are also not working, as well as the Hamburger button.

Let me know if there is anything else I can do.

Comment 6

[Mathew Hodson]

(In reply to Jeremy Swinarton from comment #4)

Hi Rares, would you or someone else from QA be able to try to reproduce this? We thought this was fixed by bug1984199, but it still seems to be happening in some cases. Thank you!

Code fix was in big 1984342

Comment 7

[Pedro [:pedrov]]

Bug 1993804 is also relevant, likely even duplicate.

There's a bit more info here in the direction of the bug not requiring dragging as earlier thought, and since 144.0, all the freezes I ended up running into didn't even involve tab dragging. For example the most recent encounter was triggered by pressing the "New Tab" plus sign, so dragging debugging doesn't seem to do much here, and my console is also empty when looking for issues, although I can't intentionally reproduce the problem with all the relevant preparation, as it happens maybe once or twice a week for me.

Using Wayland + Flatpak, can't even remember the last time I used pinned tabs, so those are not necessary.
Mouse interaction appears to be required based on frequently used Ctrl+t, Ctrl+w shortcuts never causing issues, while various mouse operations like even closing a tab with middle click did.

Suspecting that higher load is making it more likely to encounter this problem. In my case that "naturally" builds up by Firefox not being closed if not necessary, and some operations tend to get rather slow after a few days, getting to the point of opening a new page causing stutter/interruption in media playback in other tabs.
I seemingly always run into this issue when there's always significant uptime, so the UI is already somewhat slow.

As a loosely relevant past experience, I would assume that a high performance host with low I/O wait times may not encounter this issue. I've seen most of this kind of broken state on a lower power host doing a lot of I/O over NFS with an HDD target, which tends to make various parts of the system freeze for even several minutes, with some programs not even surviving that. Firefox didn't crash in such cases, but exhibited various oddities like a window mostly freezing (kind of responsive but mostly frozen UI like here), or not being able to open new pages anymore, just showing blank pages without even connecting to servers.
Moving away most heavy NFS-using tasks solved most of these issues, and upgrading to a high performance host pretty much made them disappear, but likely the fundamental issues (race conditions?) are still around in the code, not likely to be encountered in near-perfect conditions.

Comment 8

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

The severity field is not set for this bug.
:mak, could you have a look please?

For more information, please visit BugBot documentation.

Comment 9

[fokko.perton+bugzilla]

Attachment: Bug from dragging to a new window.

I was just using FF as normal, and noticed that I can even get the bug to happen with just 1 tab open by dragging a tab out of (an otherwise unbugged) window. As can be seen from the screenshot, the tab is (partially) over the "+" tab button, and the URL/Search bar are non-responsive to clicking (still works with f6 though).