I’ve had this problem for a very long time but now I thought I should check with the community how to solve the problem.
In several applications, that I thing uses GTK gives a very wide scrollbar, e.g. qpdfviewer, MasterPDF, games.
I’ve tried to set the config file ~/.config/gtk-3.0/gtk.css and adding the text:
scrollbar, scrollbar button, scrollbar slider { min-width: 10px; min-height: 10px; }
but it did not make any difference.
Any ideas?