|BTW, some additional symptoms which may or may not be relevant....|
I tried a different media stream, this time a slideshow in the Firefox browser of some pictures my son sent.
When the Firefox Window was full screen, it could not be dragged over to the other monitor.
However, if I made it less than full screen, I could drag it over OK, "and" the slidehow went with it just fine.
So it's not all images that disappear, just video ones.