While using Geany in Gnome I found that sometimes code navigations was slow, changing to a new line o making selections were reflected after aprox 1 second on the screen.
Just opening a file and navigating doesn't trigger this problem, after editing, saving or selecting some text the issue shows up.
I have other setup of Stretch with MATE desktop and the problem isn't there.
I also compiled a clone of the geany github repo and I was able to reproduce the problem (in Gnome).
(I filed a bug report to the debian package)