[Bug] top-most tab in the tree 'sometimes' 'slides' 'under' the "Tree Style Tab v" sidebar header

[jmbreuer]

Abstract

"Sometimes" (see below), the top-most tab (just noticed, sometimes even two) will "slide under" the "Tree Style Tab v" sidebar header.

Steps to reproduce

1. Start Firefox with clean profile.
2. Install TST.
3. Use it, possibly use it A LOT. Have a number of windows with a number of tabs each.
4. In some windows, the top-most tab will slide "under" the "Tree Style Tab v" sidebar header
   4.1 this is not related to the number of tabs / the tab sidebar running out of (visible) space
5. Pointing to the barely visible bottom line of the hidden tab will make it slide down and become clickable

Expected result

The top-most tab should not 'auto-hide' / 'auto-slide' under the tab bar header.

Especially, this behavior should not (appear to) be random.

Actual result

"Sometimes", the top-most tab (just noticed, sometimes even two) will "slide under" the "Tree Style Tab v" sidebar header.

While trying to make a screen recording of this issue, quite by accident I discovered that toggling the Bookmarks Toolbar (which I usually have set to "only show in new tab") would "fix" this behavior... for a very short time, switching between tabs soon caused the "sliding under" behavior again. It does only seem to happen with the Bookmarks Toolbar either always visible or "Only Show on New Tabs", but in the latter case also when a tab is selected where the Bookmarks Toolbar is not shown.

TST-SlideUnder-2026-02-01_08.14.10.mp4

Environment

• Platform (OS): gentoo Linux x86_64
• Version of Firefox: LibreWolf 147.0.1-3 (the "-3" means its the third revision of the gentoo package; I've observed this behavior for months at least - way longer than this specific version of LibreWolf, so it should not matter too much.)
• Version (or revision) of Tree Style Tab: 4.2.9

[piroor]

It looks to be a designed behavior around new tabs. Could you try the config?: TST options => turn on "Unlock Expert Options", and "Appearance" => "Suppress visual gap of the sidebar contents produced by temporarily shown/hidden toolbars on following cases" => turn off "Blank new tabs on Firefox 85 and later".

[jmbreuer]

Could you try the config?: TST options => turn on "Unlock Expert Options", and "Appearance" => "Suppress visual gap of the sidebar contents produced by temporarily shown/hidden toolbars on following cases" => turn off "Blank new tabs on Firefox 85 and later".

Thank you!

That option was turned on, I've turned it off and I'll know shortly if that makes a difference.

[jmbreuer]

... I'm not seeing it again so far - guess that was it! Thank you so much.