Not sure, they do say: Application authors relying on Qt/QML, GTK3, XUL etc. should not be required to perform additional porting as we will work on provid

Not sure, they do say: Application authors relying on Qt/QML, GTK3, XUL etc. should not be required to perform additional porting as we will work on providing Mir integration for the most prominent toolkit choices. Maybe they've just decided to take Qt first, and that's why there's already a project for this. I'm a bit confused, I thought GTK+ did its rendering through Cairo, which would mean that a Cairo backend is what's needed. Not 100% sure about this though, I don't track GTK+ at a low level these days.