Scroll bars missing with Magic Mouse
System Preferences → General → Show scroll bars: Always
When using a mouse OSX hides scrollbars by default. They appear only when you use the wheel motion. Once you are scrolling you will be able to see the scrollbars and they will be hidden momentarily once you stop scrolling. Also when you hover on them you can interact with the scrollbars.
Or you can disable this and make them visible by changing scrollbars options from the settings