Would be 5⭐ if it was possible to set a minimum height for the indicator. As of now, the height scales inversely with the number of tabs, and it becomes too thin if the tab list is long (feature request! 😇).

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.

It is usefull to an extend, but most people using TST want to have a lot of tabs open so it would be nice if the extension itself made it more clear what tab you are in (having thicker line on a tab you are in and maybe replacing a coloured line with a pattern)?

Not entirely accurate, but still usefull

Works fine.
My only grip with it is that I particularly use the scrollbar in TST on the right side and I can't choose the marker to be there.