Hover effect on vertical tabs is disturbing for the user

[PatriceGr]

On version 138.0a1 (2025-03-25), the hover effect on vertical tabs is almost unusable. In fact, if you have different groups of tabs, open or closed, and tabs outside a group, you sometimes get an offset of some pixels, and lose the focus you had when clicking on the particular tab icon.

In short, I'm not a fan of this implementation at all. I hope to see a fix soon, but I wanted to point it out so that we can make a bit of progress on functionality. But I think you're already aware of what's happening in use.

Regards

[siffemcon]

That is not happening for me on 138.0a1. As you're using a test version which is prone to breakage, reporting possible [bugs](https://bugzilla.mozilla.org/enter_bug.cgi/) is to be expected.

[PatriceGr]

I use Firefox Multi-Account Containers. I think you need to look at extensions on this side that alter the rendering of tabs in the tab bar by adding pixels at the top and bottom of such tabs. I don't know if I'm making myself clear, sorry.

I will go make a bug report on the extension.

[PatriceGr]

I have make a bug report here: https://github.com/mozilla/multi-account-containers/issues/2737

Regards.

[PatriceGr]

The bug report should not refer to the extension mentioned, Firefox Multi-Account Containers, but is indeed a bug related to tab grouping even in Firefox Nightly. I tried it with a blank Nightly session, and added a few tabs, and a group of tabs, and indeed, there's a disturbance between the rendering of folded favicons, and unfolded tabs with that famous hover effect. It's ugly and disturbing.

In short, it's not up to scratch.

[PatriceGr]

Here is the bug in a image (two screencaps). I have drawn red dotted lines to visualise the problem.

[PatriceGr]

Bug report made here : https://bugzilla.mozilla.org/show_bug.cgi?id=1956394