So you want that tab scrolling is enabled but doesn't work? - @leocg

No.
I want "Enable tab scrolling when many tabs are open" to be turned on.
When turned on there are at least 4 ways to scroll:

click on the scroll bar above the tabs to jump (i.e. if you are at the left most tab and click above the top of the right most tab, you jump to the last tabs) drag the handle of the scroll bar above the tabs hover the mouse over the tabs and use the scroll wheel click on the left or right arrow on the sides of the tab bar

I can't stand method 1. and I think the most straightforward way would be to disable/hide/remove the scroll bar above the tabs (and thus method 2. as well) but I want to keep method 3. and 4. intact

Actually, it seems as though it's occured again. This is on a completely new profile directory; I changed some settings and flags to what they were originally and saw the problem has occured again. I then reset all my settings and it still occurs.

I just discovered the mistake. It has to do with reducing the size of the screen (top right rectangle). Matter thus settled.