I recently upgraded my Apple Macbook Pro operating system from Mountain Lion (10.8.x) to El Capitan (10.11.x) and instantly hit an annoying, unanticipated change to basic functionality - the maximize window button now makes the current window full screen instead of simply filling the maximum dimensions of the viewport. This issue is a real pain on dual, vertically stacked screens as in my setup of laptop at the bottom with large display above. When clicking maximize (now full-screen) within the bottom monitor you lose the dock so can't easily click between apps or see what is open. I generally use the bottom monitor for web development and admin and the top one for testing and web design.
The solution to this problem is to simply hold the 'alt' button when clicking maximize.
Here is what the default window controls look like:
If you hold the 'alt' button down on the keyboard the maximize window button changes to the common window functionality of maximizing the current window to fill the maximum dimensions of the viewport.
This issue (feature) is present from OSX Yosemite (10.10.x) onwards.