OS X has a variety of features that help people navigate, view windows and other items, as well as aid people with visual and hearing disabilities. Some of these features are relatively obscure, and need to be researched to enable, but others are more easily enabled. One of these, which has puzzled new OS X users is when the desktop seems to be larger than the extents of the screen, regardless of the resolution being used. This results in the screen moving around with the movement of the mouse.
CNET forum member "hbgowest" writes:
"For some odd reason the entire screen resolution has increased and is now too big for the actual monitor. So now every time I move my mouse the ENTIRE screen inside my monitor moves around with it...I have tried adjusting the display size thinking that may have been the problem, but it is at 1680 x 1050 which is the largest it can get on this computer."
This feature is the exceptionally useful (but sometimes overlooked) screen zooming that allows you to keep the current resolution but magnify the desktop. It is available in the Universal Access system preferences, but also can be inadvertently enabled by pressing "⌥8". If the zoom setting is at any value above 0, the screen will automatically zoom in at this point, and the zoom value can be controlled by pressing "⌥=" or "⌥-".
If you see the zoom enabled, you can easily disable it with the above key combinations. Be sure to check out the other features of the Universal Access system preferences, since they are also activated by hotkeys and cause similar oddities to occur.