Ahh, I wonder ...
@nyamatongwe is the [patch](https://gitlab.gnome.org/GNOME/gtk/-/issues/5397) to Scintilla needed for Scintilla to actually provide that format if its requested, not just to advertise it? If GTK advertises it Scintilla doesn't know how to provide it?
Or perhaps GTK should translate `text/plain` to `STRING` and `text/plain;charset=utf-8` to `UTF8_STRING` in requests, otherwise apps won't recognise the format when its requested since they didn't advertise it.