Have you ever noticed that wide grey bubble in the top right of some of your Mac application windows? When I first moved to Mac it took a while before I even noticed it. Its actually quite handy.
The button, sometimes called the ‘lozenge’ is used in applications that have a ‘Hide Toolbar’ option in their ‘View’ menu.
In Finder the button will hide both the toolbar and the sidebar. If you command-click the button it will modify how the toolbar is displayed, cycling through variations of words and buttons, buttons only, words only and various different sizes of buttons etc.
Another handy use of the lozenge is installing an app. When doing so you are often presented with the window that has the app icon. Sometimes there will be an Applications icon that you can simply drag the app icon into in order to move the app to the Applications folder. When this isn’t the case people usually think they need to open another Finder window in order to drag the app to the Applications folder. However, the lozenge can save you this trouble. Simply click on it and a sidebar will appear and you can drag the app to the Applications folder listed there. Nice!
Some other applications that have the ‘lozenge’ include:
Mail, Preview and Adium.