Ever since I upgraded to OS X el Capitan (I'm behind the curve, I acknowledge), the little green button in the upper left-hand corner of all my apps and windows now does unexpected tomfoolery. Instead of maximizing or re-sizing the windows, like it did in all previous versions of OS X, the button takes into full screen mode. Suddenly everything that was maximized became uber-maximized, and it annoyed the bejeezus out of me.
Here's the fix. If you want to call it that.
Old school is better, in my opinion: just Option-Click on the green button. BAM, you're back to the comfortable old days, when green buttons just meant maximize.