GTK+ bug above, but migrated to Gitlab: https://gitlab.gnome.org/GNOME/gtk/-/issues/615
Related Scintilla bug with a patch attached: https://sourceforge.net/p/scintilla/bugs/2196/
Related mailing list message: https://groups.google.com/g/scintilla-interest/c/a8Scl0TimkY/m/vu3culeRBwAJ