[regression] QDockWidget doesn't resize correctly on 2nd screen

Using the mainwindow demo, if I float a dock widget and try and resize it on my left screen it shrinks to a minimum size (small square) and can't be enlarged. If I float it onto the right hand screen it can be resized, regardless of the screen where the mainwindow is displayed. The main window itself, and file dialogs can be resized on the left screen. The same demo works correctly when compiled against Qt 4.5.1.

I am using the NVIDIA driver with the following in my xorg.conf:

Option "TwinViewOrientation" "LeftOf"

ie. I presume the right screen is the primary one.