I should have said, this didn't help. I wasn't sure what "reset" meant, but repeatedly closing the window and opening it with the button, Ctrl + Shift + F5, Ctrl + Shift + Click, didn't make any difference in the window (it looks as above).
Also, I cannot seem to resize the window. I thought I might have done so by accident, but I cannot get the cursor to change to the resize cursor.
Also, I use PDN in full screen mode but using it in a floating window didn't make any difference.
I guess I could restart PDN. I have a bunch of images open...