NOTE: You must include the UNITS with the width. I've found the following that work:
px, in, mm, and cm.

The only thing wrong with this add-on is that the unit selection isn't built-in, and the add-on's page doesn't really mention this crucial bit of info for this add-on to work.

On my 4K 32-inch monitor, I use "5px" in the addon's Width field in its options -- you can change the number and the units to whatever will work, as wide as you want. You can choose any color you want, too.

(BTW, this may be obvious, but you do need a scrollable tab tree (with lots of tabs) for this add-on's marker to display.)

Nice. But this ( https://github.com/piroor/treestyletab/wiki/Code-snippets-for-custom-style-rules#show-scrollbar-in-the-tab-bar-rightside-1362 )
should be in the description.