Our LibreOffice gtk2 vclplug inherits from our generic X11 vclplug and so in lots of places we just continued to use our historic X11 vclplug for various things, one big example being clipboard support.
To do the same with the gtk3 vclplug would work for the case where gtk3 is backed by X11, but not if backed by wayland. So we needed to implement cut and paste with the gtk3 apis.
X clipboard/selection/cut and paste is errr... "tricky", so it was a bit of a death march to drag myself to the keyboard to go about this. But it turns out the gtk clipboard apis are really good and its reasonably easy to get it up and running. So the LibreOffice gtk3 vclplug now has clipboard support.
Last major thing is to get gtk3 gstreamer integration working for video playback and then it's mopping up territory.