Now, if you have problems turning of fullscreen, cause minecraft sets it to default whenever you launch minecraft, just go to the minecraft folder in finder (mac) and in there you should find "options.txt". Double-click it and you should be able to edit it. Now, scroll down until you find "fullscreen:true". Delete the "true" part and type "false" in. Don't forget to save. And there you have it. Next time you launch minecraft fullscreen will disabled (can be enabled at any time).