Thanks for your prompt comment **elextr**. I build from source (on Fedora 29): `./geany -V` `geany 1.32 (built on Mar 19 2019 with GTK 3.24.1, GLib 2.58.3) ` (GLib 2.58.2 updated to 2.58.3 last night!). The problem is still there.
Going to study GTK-3.24.1 behavior with dead keys in edit windows (as it seems the easiest place to dig in).