GTK: Avoid warnings about unused parameters
GTK: Don't forward calls to NULL scrollbars
Incorrect font size in auto-completoin list on GTK >= 3.21
Geany backfills with spaces when typing within virtual space
Fixed in Scintilla 3.6.7 imported in [18db459632b62aa95bc3699a9e38c914d1220e79]
Support out-of-source build in GTK makefiles
I'm not sure what you tried exactly, but attached patch sets a few style details...
I'll try and see if it can't be fixed possibly the same as you did for GTK3 using...