http://wordpress.org/extend/plugins/photonic/
There seems to be a JavaScript conflict wit this plugin that causes the drop down to auto-scroll to the bottom of the list. When using the mouse scroll wheel it’ll make it not possible to select items in the middle of a long list of categories. There only seems to be two solutions. Drag the scroll bar to scroll through the list of categories or deactivate the plugin and find an alternative solution.
NOTE: This FAQ was written based on Photonic 1.22