RawMotion events not delivered when a button is held down

[Philipp Reh]

I have a client application that receives XI_RawMotion on its RootWindow.
This stops working if I hold a mouse button down and the mouse is
inside the client's window.

Apparently, the server generates two RawMotion events per single motion.
If everything is normal (the button is not held down) one of these events
is delivered in the if(grabbed) part of ProcessRawEvent, while the other
is delivered in the else part (and sent to every root window).
If the mouse button is held down both of them are delivered in the 
if(grabbed) part, and none of them reaches the client anymore.

What could I be doing wrong? It doesn't seem to matter whether I grab the
mouse myself or not. I also couldn't make enough sense of the server code to
actually see if there is a bug or not. Can someone please look into this?

Cheers,
Philipp