Screen flashes white on first movement of bottom-right portlet, using FF2

On the first movement of the bottom-right portlet, the screen flashes white.

Steps to reproduce:

1) go to the Uportal page from the daily build site:
http://build.fluidproject.org/fluid/sample-code/reorderer/portal/portal.html

2) Using the mouse, attempt to drag the bottom-right portlet. Notice the screen flash white.

This does not occur with keyboard dnd, or if you move another portlet first.