OSX is calling the operator twice (once right after the windows state changes, before the operator even ends).

For the records, this patch produces the follow printing in OSX:
"wm_window_fullscreen_toggle_exec CTX_wm_windows(C) is not valid"

In other OSs I suspect the result will be:
"wm_window_fullscreen_toggle_exec is valid!"

This patch doesn't need to be committed, but I hope it helps to illustrate the source of other issues (fullscreen in OSX is having troubles to deal with mouse click properly).
This patch also helps to alert about the issue of double op being called (it was really hard for me to finish my upcoming patch on fullscreen editor with that bug, so was this committed it would have saved me plenty of time).