The best way to explain it is... You know how when you're at the dock and you put your mouse over something and it magnifies it? It doesn't do that for me, I have magnification turned on yet that feature doesn't work unless if I click on an app then it will magnify until I stop clicking it. It's happened to me in games too. I can't move the camera/mouse unless if I click first.
How do I fix this?
iMac, OS X Yosemite 10.10.3