1993806 - Vertical tabs "Expand sidebar on hover" activation timeout is too low, causing noisy behavior

Status: RESOLVED DUPLICATE of bug 1983125

(Firefox :: Sidebar, defect)

Description

[Evan Arnold]

Attachment: vertical_tabs_left_visually_noisy.mkv

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:143.0) Gecko/20100101 Firefox/143.0

Steps to reproduce:

1. Enable vertical tabs
2. Enable "Expand sidebar on hover"
3. Ensure your Firefox window is floating and not full screen. (Or, have multiple adjacent monitors where you can move your cursor.)
4. Place your mouse cursor in the middle of your Firefox window.
5. Quickly move your mouse cursor to somewhere outside the window, passing over the collapsed vertical tab bar.

Actual results:

On Windows 10 and Windows 11, the vertical tab bar will expand, no matter how fast your mouse moves. (On Linux Mint, I was able to move the cursor fast enough that the tab bar did not open.)

Bonus bug: If the tab bar is on the left side of the window, the tab bar will immediately close again after it finishes opening. But if the tab bar is on the right side of the window, the tab bar will stay open and not close automatically.

Bonus bug 2: If the tabs are on the right side, if you re-enter the window with collapsed tabs and put your mouse in just the right spot so it barely overlaps the fully-opened tab, you can get the tab bar to repeatedly open and close, which I dubbed "flapping."

Expected results:

When moving your cursor over the tab bar and out of the Firefox window, which happens often during regular computer use, I would not expect the tabs to expand. I only expect the tabs to expand when my cursor lingers over the tabs for a longer (intentional) period. Further, if my cursor has exited the window, I would expect the vertical tabs to not remain open unless my cursor is actively hovering.

Weirdly enough, I can get the vertical tab on the right to close if my mouse exits the window after hovering on the tabs long enough for it to fully open.

I've noticed multiple annoying symptoms, so it could be filed as multiple bugs, but I believe the root cause is the same.

Comment 1

[Evan Arnold]

Attachment: vertical_tabs_right_stuck_open.mkv

Comment 2

[Evan Arnold]

Attachment: right_tabs_closing_and_flapping.mkv

Comment 3

[Evan Arnold]

I have attached three videos that show the behavior I'm talking about, and why it's very visually distracting.

• vertical_tabs_left_visually_noisy.mkv - shows how often the tab bar opens and closes while just moving the cursor around different windows.
• vertical_tabs_right_stuck_open.mkv - shows similar behavior, but shows how the tab bar remains open if the tab bar is on the right side of the window
• right_tabs_closing_and_flapping.mkv - shows how it is possible to leave the window and have the tab bar close as expected, but then also shows the "flapping" behavior if the mouse is in the correct spot to re-trigger the sidebar opening.

I believe all of these bugs could be fixed if the tab sidebar "hover timeout trigger" is increased, so hovering takes only a few milliseconds longer to open the tab bar. If my mouse cursor simply leaves the window it should not expand the tabs unless my cursor is moving very slowly.

Comment 4

[Evan Arnold]

To clarify, I was able to reproduce all of these behaviors on Windows 10, 11, and Mint.

Comment 5

[BugBot [:suhaib / :marco]]

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 6

[Dana Lucaci, Desktop QA]

Reproduced with Firefox 143.0 (2025-09-08) on Windows 10.
Marking this issue as new for engineering input.
Thank you!

Comment 7

[Kelly Cochrane [:kcochrane]]

We added a pref for this in build 145 ("sidebar.animation.expand-on-hover.delay-duration-ms"). It's set to 200ms by default, but you should be able to adjust that to your preference to help with expand sensitivity.

Comment 8

[Evan Arnold]

That's great!
I am happy to hear that. That should technically solve this issue, as an optional preference.

However I am wondering if this should be re-opened (or maybe a new bug should be opened?) as the sidebar remaining stuck open when on the right side feels like it's a different bug, even if the increased hover time does solve the issue.

In other words, with a short hover time, having the side bar open and close as the mouse moves around is expected behavior. Being able to change the hover delay is a great feature.
However, regardless of how long the hover delay is, the side bar should always close again when the cursor leaves it.

If you don't want to re-use this bug for that then I can open a new one that's more specific.

Comment 9

[Kelly Cochrane [:kcochrane]]

(In reply to Evan Arnold from comment #8)

That's great!
I am happy to hear that. That should technically solve this issue, as an optional preference.

However I am wondering if this should be re-opened (or maybe a new bug should be opened?) as the sidebar remaining stuck open when on the right side feels like it's a different bug, even if the increased hover time does solve the issue.

In other words, with a short hover time, having the side bar open and close as the mouse moves around is expected behavior. Being able to change the hover delay is a great feature.
However, regardless of how long the hover delay is, the side bar should always close again when the cursor leaves it.

If you don't want to re-use this bug for that then I can open a new one that's more specific.

Hmm that sounds similar to bug 1985692 which is already being worked on.

Comment 10

[Evan Arnold]

Oh sweet, yep! That definitely looks related. Thanks! This can stay closed :)