Vertical Tabs scrolls to top when pinned tabs are present and "expand on hover" is enabled

[jandark]

Description:
When using Firefox vertical tabs with the "expand on hover" feature enabled, selecting a lower tab while the sidebar is collapsed causes the tab list to scroll to the top — especially when pinned tabs are present and the total number of tabs exceeds the vertical screen height. This makes it harder to track which tab is active and breaks the user experience.

Steps to Reproduce:

1. Enable Vertical Tabs in Firefox.

2. Enable Expand on Hover feature for the vertical tab sidebar.

3. Pin a few tabs (e.g., 2–3).

4. Open many tabs (more than what fits the vertical screen height).

5. Collapse the sidebar (so only icons are shown).

6. Click a tab located near the bottom of the list.

7. Sidebar scrolls to the top automatically, instead of focusing the clicked tab.

Expected Behavior:
When a user clicks on a lower tab, the sidebar should auto-scroll or remain focused on that tab — without jumping to the top of the tab list.

Actual Behavior:
The sidebar scrolls all the way to the top after selecting a lower tab, hiding the selected tab from view. This behavior only occurs when pinned tabs are present and the sidebar is in collapsed mode with hover-expand active.

Browser Version:
Firefox 138.0.1 (64-bit)

Platform:
🖥️Windows 11 24H2

Video Example:
📹 Video Demonstration (Google Drive)

[AyeBraine]

This killed the vertical tabs functionality for me! I tried it, abandoned it, and then found the solution — remove all pinned tabs. With pinned tabs removed, the tab list reliably maintains it position for me (I routinely have dozens of tabs open at all times). I would very much like to use vertical tabs, but this bug doesn't let me.