universal access zoom feature finally fixed2007-10-31 17:53:50 -0700
Prior to Mac OS X 10.5 (Leopard), there was a bug that made using the Universal Access zoom feature (accessible by default with Option-Command-8) useless in a multiple display configuration in which the virtual heights of the arranged displays differed. As soon as you activated the zoom feature, the portal created by your displays would lock essentially to the height of the primary display, making it impossible to scroll to the portions of the virtual screen that mapped to the higher or lower displays, and in some cases you couldn't even see the menu bar. Basically, you could pan horizontally across your virtual screen, but not vertically.
This might not make any sense to you if you never tried this yourself, but they fixed this in Leopard and I'm happy about that.