There appears to be a bug with the overview control - it doesn't always display the diagram if the diagram doesn't start at the top-left corner of the view port.
In the attached example, the page loads with the diagram zoomed out and positioned in the centre of the view port. The overview shows nothing.
Now if you keep moving the diagram closer to the top left of the view port and clicking the 'Refresh Overview' button, you'll see the top-left circle begin to come in to view on the overview control.
Since this is a little hard to explain, I've also attached some screenshots with various scenarios of the overview control working and not working. Images 1, 2 and 3 show it working fine. Images 4 and 5 show it not working.