When you click the green zoom button at top left of a window, that window goes full screen. It hides the menubar, it hides the window’s bar , it hides the window’s bottom frame. That’s not usually what we want to do. If you hold the Option key down when you click a window’s green zoom button the window will still zoom, but stop short of hiding the menubar and the title bar and the bottom frame. This is how the green button used to work but it changed in macOS Yosemite. As usual, the Option key is your friend. Try it and see.