Ah, I misremembered how it happened. In the Export Image window, I click Export, and a small window appears over it with some JPG options, and the Export Image window becomes inactive. If I then click the Export Image window (which I might do if I leave it and then return), the Export Image window comes to the foreground, hiding the small options window, but it is still inactive.

I just realized that there is a way out of this bind, because there is a shadowy icon in the dock representing the small options window, and if I click on it the options window comes to the foreground.

Also: the Export Image window is not modal. It does cover the main image window, but if I drag it out of the way I can edit the image in the main image window. Do we really have the same version (2.8.10p2)?