There is some bug with Overview plugin in last versions of Geany. When the plugin is on, Geany's text starts shimmering, i.e. some lines are doubled, then set properly. I couldn't find a regularity of this, except for that with a code (and syntax highlighting) the shimmering occurs more often than with a plain text.
When the Overview plugin is off, the shimmering disappears too.
See the picture attached.
I use also Git monitor plugin. It doesn't produce the shimmering by itself and doesn't influence at all. It seems Overview plugin is responsible for the shimmering.
Debian 10, Xfce.