Thanks for your help, Juan, Zocky and Harry. In fact, Zocky's post above is especially enlightening.
I do also see the issue where sometimes an image fails to load (ie you choose it in the open-file dialog and nothing happens), but I had thought it was unrelated. I now notice that when this does happen (~50% of the time, but I haven't worked out a pattern), then when I retry to open the image (which tends to succeed) then the tab on the top-left of the image *is* blue (normally grey) and the mouse will disappear over this image. Any further images opened in this session have the same problems (blue tab, no mouse). If I manage to get a file to open first time, then it's fine, and generally other files opened in the same session are fine. So, basically, it seems to depend on what happens to the first image opened.
I can give screen shots or casts if that would help, but not for a couple of weeks as I have a holiday. Anyway I have an (albeit tedious) workaround for now, which is just to quit and reopen PI whenever an image fails to open.