In OS X Lion, Apple has increased and enhanced the operating system's scrolling behaviors, with one aspect being that when you scroll beyond the extent of the window contents, the window will show a textured background followed by bouncing back in a smooth fashion.
You can see this behavior clearly in Safari, where if you scroll up and down quickly, when you get to the end of the page you will see the textured backdrop appear, followed by the page moving back to cover this up. This behavior also works in the horizontal direction, and should spring back to the main window contents when scrolling has stopped; however, there may be times when the behavior conflicts with some applications, causing unwanted behavior.
Apple Discussion member Bryan S. writes:
This is just driving me crazy. Installed Lion and all seemed well. I have a MPB 2008 with a Magic Mouse.
When I am browsing in Safari (and anywhere else for that matter), if I scroll down and move just a little bit to the right or left then the page "giggles" back and forth and bounces on the edge of the screen. Extremely annoying. If I go to Mouse Preferences and turn off gestures, I STILL get the behaviour. If I go to UA, fourth tab "Mouse and Trackpad" and click on Mouse Options and deselect "scrolling" then I don't bounce around, but I can no longer use the MM to scroll.
This type of behavior happens when two of Apple's scrolling behaviors conflict with each other. The first is the bounce-back that happens when you scroll beyond the extents of a page's contents, and the second is the inertial scrolling in the OS where once you start a scroll action the page will continue moving in that direction and slowly slow down, instead of halting immediately. When the jiggling problem happens, the system's bounce-back behavior invokes the inertial scrolling that sends the page past the extents of its contents, which in turn causes it to bounce back again.
This conflict should not happen, but if it does then the workaround for it is to disable one of the automatic scrolling behaviors. Unfortunately the ability to scroll beyond the extents of the page cannot be disabled in the system, but you can disable the inertial scrolling in the system. This is available in the Universal Access system preferences by going to the Mouse & Trackpad section and clicking the Trackpad Options button. In the window that opens, change the scrolling behavior to be "without inertia" and click the "Done" button.
This setting change should prevent jiggling behaviors from occurring; however, it does come with the drawback that inertial scrolling will no longer work.
Other options that might help prevent this behavior are to change the scrolling speed of your mouse, and also adjust the window size if it only is happening for a specific window.