Hi, I'm also having this issue whenever FCPX is running. On a Macbook Pro with GeForce GT 750M. As soon as I quit FCPX behavior is back to normal.
After reading through this thread where people were having similar issues but with VLC, I'm wondering if there's some rendering bug with Nvidia cards maybe? Like that thread, when I use a theme with no blur, the issues resolve.
In that thread it was suggested to contact VLC devs but it seems that either Apple or Alfred might be where the focus should lie (probably Apple, frankly).