You have a lot of content in this area.
I barely use mouse wheel scroll, but I shouldn't have to.
Tried dragging the black oddly rendered bar down and it does not move or indicate where in the page I am viewing.
If I switch to another application window, then return to this one, I notice the bar tries to update to the location it should be. This is a good clue.
Using the latest Firefox 63.0.3 on Windows 10.
I think the scroll element scrollbar-2rkZSL looks better at 16 to 20 pixels.
After messing around with the source. It's a huge amount of code for what I'm familiar with.
No conclusive answers, but check out areas with the following snippets:
- _queueFullUpdate or _queueLightUpdate
- setFullScrollerState or setLightScrollerState
These are frequently expressed with short-circuit logic, so if the former was FALSE...
- needsScrollbar && updateScrollBar()
Could also be related to the post about the laggy Scroll Bar in the Content area.
To be fair, that one does eventually update.