As reported in ticket:1870#comment:7: I've had to remove --sync-xvfb=15 from my xpra launch args. Otherwise the mouse doesn't seem to be able to interact with any windows, I can move, resize etc (all window manager type stuff), but clicking on buttons does nothing, and mouse-overs are not triggering, so I guess the apps are unaware of the pointer location.
This regression was caused by r16702 and is fixed in r20582.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1968