Thanks for your reply.
1. the right mouse button is held not clicked
I am not holding the right mouse button. The action happens during a normal right-click. I do not have issues with native macOS apps, Firefox, etc. in regards to right-clicking -- this only happens with Geany.
2. while its held the cursor moves to the top menu item (undo) due to shakey hands, cosmic winds or something else
Not cosmic winds -- I'm in an ultra-secure mega-shielded location đ
It's disappointing that this happens because Geany has been my favorite coding environment for years -- mostly on Linux. Now that I'm on macOS, I'm really hoping to keep using Geany, although renaming the conf file directory is a small price to pay. đ