I have noticed on several Macs like Mac Mini's and a MacBook Pro running OSX versions all the way up to El Capitan that left mouse or trackpad first time clicks sometimes don't register when clicking on the "x" to close the window or also on tabs.
I noticed that clicking anywhere inside the active window first will wake up the "x" or tabs.
This looks like it may be a bug that was never reported.
Does anyone else have this issue and is there a setting I can set?
* Update *
I'm now using El Capitan for quite a while and Apple has not fixed this issue yet.