If you open Xmaster, and switch to another app while it's starting, once it's running it takes over the whole screen, but doesn't receive the mouse & keyboard events, because it's running in the background. You have to blindly switch into it (for example by opening a terminal and typing 'open -a Xmaster').
There are two possible solutions:
1. switch to Xmaster as soon as it's running
2. check if we are in the background and disable the display when necessary