Vertical tabs `Expand sidebar on hover` causes layout shift.

[ELowry]

What happened?

When using vertical tabs and enabling the Expand sidebar on hover option, entering the sidebar with the cursor (hovering) causes a margin to appear next to the sidebar, and a layout shift for the main content:

Waterfox.v6.6.12.2026-05-11.14-47-54.mp4

This is visually unappealing, but can also cause performance and content issues with websites that hook into viewport size changes using JavaScript.

Reproducible?

• I have checked that this issue cannot be reproduced on Mozilla Firefox.

Version

6.6.12

What platform are you seeing the problem on?

Windows

[ichirou2910]

Can also encounter this on Linux, but in version 6.6.14 instead. I downgraded to 6.6.13 and the problem disappeared.

[ELowry]

As of version 6.6.17 (on Windows) it is happening in Incognito, but not in normal tabs.