The scrollbars in OS X Mountain Lion and Lion are hidden until activated by use, this is the new default behaviour that works very well if you primarily use a trackpad with your Mac.
Set Scroll Bars to Display All the Time in Mac OS X
This will return scroll bar behaviour to that which existed in Mac OS 10.6 and prior, that is, being always visible:
- Launch System Preferences from the Apple menu
- Click on General
- Look for ‘Show scroll bars’ and select the radio button next to ‘Always’
Scrollbars are instantly visible with this change.
These settings work in OS X 10.7, 10.8, and beyond. Expect this to be the new standard as iOS and OS X continue to share more features.