GTK3 supports "Smooth scrolling" by clicking on the scrollbar slider and holding the mouse button until it becomes "thin", which enables a mode that allows very precise scrolling.
VCL scrollbars don't support this currently, and they also don't display the "scroll up" / "scroll down" buttons with GTK3 so there is no way to do precise scrolling.
Steps to Reproduce:
1. left click on scrollbar slider
2. hold button for some seconds
3. notice no change in scrollbar behaviour
3. change in scrollbar behaviour to precise scrolling
User Profile Reset: No
User-Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0
FWIW starmath text scrolling area, because native gtk used, now has this in master