Just a note: I uploaded gtk3 Geany to Debian recently (1.32-2).
Also, before I realized there was already a PR in progress, I made an attempt at porting it myself: https://github.com/hyperair/geany-plugins/commit/1fd266a97b9fedb0ae463a2cf9a...
It compiles and works, but is missing the context menu at the moment. That needs to be ported to [`WebKitContextMenu`](https://webkitgtk.org/reference/webkit2gtk/stable/WebKitContextMenu.html).