We toggle a sidebar-shown attribute on the #tabbrowser-tabbox element when the sidebar launcher element is visible. This only makes sense when the sidebar.revamp pref is true. If you disable that, the attribute remains even though when the launcher element is no longer visible. This put us in a undefined state.

STR:

• with sidebar.revamp enabled (Check the "Show sidebar" checkbox in about:preferences), show the launcher by toggling the sidebar button in the toolbar.
• Confirm in browser toolbox that document.getElementById("tabbrowser-tabbox").hasAttribute("sidebar-shown") is true.
• In about:preferences, un-check the "Show sidebar" checkbox

Expected:

• The launcher is hidden and document.getElementById("tabbrowser-tabbox").hasAttribute("sidebar-shown") returns false

Actual

• The launcher is hidden but document.getElementById("tabbrowser-tabbox").hasAttribute("sidebar-shown") return true

Hi Sam, I reproduced this and have a fix ready. I’m currently waiting for my patch in Bug 1999012 to land since it touches the same function. I'll submit the formal patch for this one as soon as that's approved to keep the history clean and avoid conflicts.

Apologies kofoworola, I have already talked to John Iweh about this bug and am assigning.

(In reply to Nikki Sharpley (:nikkis) (she/her) from comment #2)

Apologies kofoworola, I have already talked to John Iweh about this bug and am

No problem at all, thanks for the update.

I have fix the bug please review it. https://phabricator.services.mozilla.com/D291830

https://hg.mozilla.org/mozilla-central/rev/72feb6cf53fe