XGrabPointer() isn't respected under Wayland
Mutter fortunately has good support for XGrabKeyboard() now. But it seems that XGrabPointer() was overlooked. So right now vncviewer can send "Win" through, but it cannot send "Win+Right Mouse Button".
I consider this a bug as it means that users still cannot fully interact with the remote system.
I have not tested XIGrabDevice(), but I assume it's mapped to the same thing in the end.