OK, it just happened again. This time it happened when I was setting the directory for a new sequence (actually changing it to a new directory for tonite, but same sequence as last night). It just locked up. I could not grab or reposition the window and needed to close it from the task bar icon.
It looks like this. My four monitors are arranged as seen in the blue squares (note resolutions) and SGP was started maximized on the lower right (primary) monitor. That is where I always run it. When I clicked to select the directory, it jumped to the position you see in the yellow and would not be moved.