Try to fix completion positioning on Wayland

With the previous trivial porting the completion widget ends up in the void.

With the second commit, it at least seems to be positioned reasonable.

But I assume just removing this is a no good idea because one need to catch that cases for good positioning on screen edges.

I still fail a bit to see why the code behaves that differently, will try to debug a bit more in the near future in this branch.

@waqar