Shutdown button isn't clickable if you hover above it before the dialog appears

When I choose System>Shutdown or click on the shutdown button on the panel, and move the mouse to the position where the button in the dialog is going to appear, before the dialog is displayed, the button isn't clickable (and it isn't displayed in 'hover' state) until I move the mouse away and back on the button again.

This happens with all buttons in the dialog (even the Cancel button) on both my laptop, my desktop system and my parent's system, both with and without Xgl.

This happens every time and is easy to reproduce (especially on slow systems):
1. Click the Shutdown button on the panel
2. Move the mouse to the position where the Shutdown-button in the dialog is going to appear before it actually appears (you have to be fast!)
3. Try to click the button

All systems are running Dapper (with the new shutdown dialog)