Here's the result of experimentation with different versions:
Fresh Ubuntu 18.04.3 VM using apt to install: * Scite 4.0.0 for GTK+ 3.22.30 (Nov 23rd 2018) --> scrollbars OK * Geany 1.32 (built 2018-01-08 with GTK 3.22.26, GLib 2.54.1) --> scrollbars *don't* repaint properly.
Fresh Lubuntu 16.04.5 VM using apt to install: * Scite 3.6.0 for GTK+ 3.16.7 --> scrollbars OK * Geany 1.27 (built 2016-04-17 with GTK 2.24.30, GLib 2.48.0 --> scrollbars OK