It's odd if it doesn't reproduce always the same, and I thought we fixed all this kind of issues long ago… But although I can't reproduce it (I guess I'd have seen this as I use the GTK3 build all the time), it might be related to using a compositing WM, that triggers less repaints and relies on more caching.
I'm afraid I don't really know where to look this time, but around the paint code in ScintillaGTK.cxx (or, less likely in PlatGTK.cxx)… maybe see if Neil got an idea, or give me more so I can reproduce it :) --- Reply to this email directly or view it on GitHub: https://github.com/geany/geany/issues/807#issuecomment-164512597
