Instead of polling for the event (with drift from calling nanosleep() in a loop...), this code:

Should use select() or poll() to wait for the fd used to communicate with the X server to have some data, then use XCheckIfEvent() to check if the data is what we're watching for.

Edit: Is 300ms enough of a delay? 10ms is often the precision OSes offer to the userland when it comes to timers, there's probably significant drift in polling every 10ms, fixing this might expose an issue with the delay being too short, but I find it unlikely for local X servers.
Edit 2: fixed incorrect statement about poll()