I've just made the following builds:
Geany 1.38 built with the latest GTK and other dependencies, including some extra fixes which I plan to release as the official macOS build on the Downloads page unless there are some problems with it: https://download.geany.org/snapshots/geany-1.38_osx-5.dmg https://download.geany.org/snapshots/geany-1.38_osx_arm64-2.dmg
Like the above but Geany and plugins are from current git master: https://download.geany.org/snapshots/geany-1.38_osx.dmg https://download.geany.org/snapshots/geany-1.38_osx_arm64.dmg
I think a lot of has changed in the GTK drawing and input handling implementations for the macOS backend so you might try if it fixes this problem.