Apple changed the way the green button works in OS X.
Starting with OS X Yosemite, Apple changed the way the green button works, in the upper left hand corner of open windows. That button causes apps to open in full window mode, instead of zooming. If you miss the old behavior, it's only a button press away.
The menu bar of open windows in OS X contains three buttons in the upper left hand corner: A red button, a yellow button and a green button. Clicking the red button causes the window to close. clicking the yellow button causes the window to minimize (the window will shrink into the Dock). Beginning with OS X Yosemite, clicking the green button invokes full screen mode.
This isn't the way it used to work. Before Yosemite was released, clicking that green button would "zoom" the window to its full size instead. You can still do that by double-clicking the window menu bar.
If you'd like to restore the old behavior, you can. Here's how.
How to restore zoom control in OS X
- Hold down the option key on the keyboard.
- Position the cursor over the green button.
- Click the green button.
You should notice that the green button changes slightly when you position the cursor over it with the option key held down. Instead of seeing two diagonal arrows, you'll see a plus sign. That's how you know you're holding down the option key.