This is driving me crazy.  With the latest OS update, you get a split screen mode when you open two windows, such as Safari and Mail app.  I don't want a split screen and the feature is just plain dumb.  It appears there is no way to shut it off, at least my searching hasn't found anything.  Everything I have read says to press the green button to expand your window once it has already gone to split screen.  Pressing the green button actually changes the window to a small window in the center of the screen and a second press of the green button maximizes it.  I was hoping someone knew of a way to permanently disable split window.