Hidden gesture to scroll to top of SMS list, or glitch?
If you're running the stock version of Android 4.3 on your S3, this is actually a feature provided by Samsung's TouchWiz. You can find the toggle for it in Settings &gt; System &gt; Motion. You can find documentation for it on Samsung's support section. It says:

Double tap to top: When viewing a long list, like your contacts list, or scrolling through a website, tap the top of your device twice to be taken back to the top of the list or page.