As Geany doesn't do anything with the scrolling or rendering, this is almost surely an issue in [Scintilla](https://sourceforge.net/p/scintilla/bugs/) or [GTK+](https://bugzilla.gnome.org/). It might also be related to some kind of Xlib compatibility mode in Wayland, unless Geany and Scintilla don't use any Xlib calls, it's probably a kind of emulation or something.