1982129 - Expand on hover intermittently stops working

Status: VERIFIED FIXED

(Firefox :: Sidebar, defect, P1)

Description

[❌OOO Until Oct 27❌ Kelly Cochrane [:kcochrane]]

It seems we've had multiple internal reports of expand on hover intermittently not working.

STR:
Enable vertical tabs and expand on hover. While hovering over the expanded sidebar, right click the sidebar and select "Turn off vertical tabs" from the context menu. Right click the toolbar and select "Turn on vertical tabs". Expand on hover is turned back off now. Re-enable it using the customize sidebar panel. Hover over the sidebar to expand it and notice how it remains expanded even once you hover outside of the sidebar.

Note that the issue goes away with a browser restart unless you take the steps taken above again.

Comment 1

[❌OOO Until Oct 27❌ Kelly Cochrane [:kcochrane]]

Attachment: Bug 1982129 - Ensure _hoverBlockerCount doesn't go into the negatives and the hover blocker is removed when expand on hover is disabled in browser-sidebar.js

Comment 2

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

Set release status flags based on info from the regressing bug 1957008

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/8432a90dc508
https://hg.mozilla.org/integration/autoland/rev/91ee1f7e3ee5
Ensure _hoverBlockerCount doesn't go into the negatives and the hover blocker is removed when expand on hover is disabled in browser-sidebar.js r=nsharpley

Comment 4

[Serban Stanca [:SerbanS]]

https://hg.mozilla.org/mozilla-central/rev/91ee1f7e3ee5

Comment 5

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

The patch landed in nightly and beta is affected.
:kcochrane, is this bug important enough to require an uplift?

• If yes, please nominate the patch for beta approval.
  • See https://wiki.mozilla.org/Release_Management/Requesting_an_Uplift for documentation on how to request an uplift.
• If no, please set status-firefox142 to wontfix.

For more information, please visit BugBot documentation.

Comment 6

[❌OOO Until Oct 27❌ Kelly Cochrane [:kcochrane]]

Spoke with :sclements on this, and we'd like to add this to the planned dot release on Sept. 2 should no regressions arise in the meantime

Comment 7

[❌OOO Until Oct 27❌ Kelly Cochrane [:kcochrane]]

Comment on attachment 9506153 [details]
Bug 1982129 - Ensure _hoverBlockerCount doesn't go into the negatives and the hover blocker is removed when expand on hover is disabled in browser-sidebar.js

Beta/Release Uplift Approval Request

We'd like to add this to the planned dot release on Sept. 2 should no regressions arise in the meantime.

• User impact if declined/Reason for urgency: Expand on hover in the sidebar could unexpectedly stop working.
• 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: STR in the description for the bug
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): 2 line JS change
• String changes made/needed:
• Is Android affected?: No

Comment 8

[❌OOO Until Oct 27❌ Kelly Cochrane [:kcochrane]]

Looks like this is still happening per :mconley. He said the hover blocker count got up to 4 when he ran into the issue, so I need to move that decrementing line back out of the conditional.

Comment 9

[❌OOO Until Oct 27❌ Kelly Cochrane [:kcochrane]]

Attachment: Bug 1982129 - Ensure _hoverBlockerCount doesn't go into the negatives in browser-sidebar.js

Comment 10

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/a6a926a52dd1
https://hg.mozilla.org/integration/autoland/rev/7248f97502b8
Ensure _hoverBlockerCount doesn't go into the negatives in browser-sidebar.js r=nsharpley

Comment 11

[[e-mail]]

https://hg.mozilla.org/mozilla-central/rev/7248f97502b8

Comment 12

[Bianca Hidecuti, Desktop Test Engineering [:bhidecuti]]

I was able to reproduce the issue with an affected Firefox Nightly 143.0a1 build from 2025-08-08 on Windows 11, while following the steps described in Comment 0 (3 out of 3 attempts).
Verified as fixed using Firefox Nightly 143.0a1 (2025-08-13) on Windows 11, macOS 14 and Ubuntu 22.04. The sidebar no longer remains expanded once you hover outside of the sidebar (3 out of 3 attempts).

Comment 22

[Dianna Smith [:diannaS]]

:kcochrane to be sure did you only want to uplift one of the patches to release?

Comment 23

[❌OOO Until Oct 27❌ Kelly Cochrane [:kcochrane]]

We'll need to apply both patches for the uplift as my second patch fixed a regression caused by my first patch.

Comment 24

[Dianna Smith [:diannaS]]

Comment on attachment 9506153 [details]
Bug 1982129 - Ensure _hoverBlockerCount doesn't go into the negatives and the hover blocker is removed when expand on hover is disabled in browser-sidebar.js

Approved for 142.0.1

Comment 25

[Pulsebot]

https://github.com/mozilla-firefox/firefox/commit/e044dcbb17fe
https://hg.mozilla.org/releases/mozilla-release/rev/26946f2420cb
https://github.com/mozilla-firefox/firefox/commit/964d2ca40b99
https://hg.mozilla.org/releases/mozilla-release/rev/f76348ebb836

Comment 26

[Bianca Hidecuti, Desktop Test Engineering [:bhidecuti]]

(In reply to Bianca Hidecuti, Desktop Test Engineering [:bhidecuti] from comment #12)

I was able to reproduce the issue with an affected Firefox Nightly 143.0a1 build from 2025-08-08 on Windows 11, while following the steps described in Comment 0 (3 out of 3 attempts).
Verified as fixed using Firefox Nightly 143.0a1 (2025-08-13) on Windows 11, macOS 14 and Ubuntu 22.04. The sidebar no longer remains expanded once you hover outside of the sidebar (3 out of 3 attempts).

Verified as fixed using Firefox 142.0.1-build1 on macOS 14, Windows 11 and Ubuntu 22.04 as well.