Thanks codebrainz, I just went ahead and rebuilt it with Gtk+2, and everything is perfect without having to mess with any CSS, etc. I cannot find any Gtk+3 theme that makes Geany usable in the limited space of a VM. All Gtk+2 themes are fine.
Here is the comparison screenshot. All buttons fit with "Room to Spare":
[Screenshot on OpenSuSE Leap 15 Beta (Gtk+2)](http://paste.opensuse.org/14174908)