Thank you for this!
I'm certain that this is, indeed, related to the WebGL canvas crashing for some reason; every time you select an item, the corresponding photos are loaded into WebGL's texture cache. My guess is that there is a problem that with freeing the cache so that it eventually runs out of memory and shuts down the canvas. That would explain why it happens after a while (e.g., once you loaded a given number of items). The bad news is that I have not been able to reproduce this so far and since we do not 'officially' support Windows 7, I can't make any promises that we'll be able to resolve this soon.
I hesitate to say this, but If you enable developer mode in the preferences, you should be able to use 'Reload' rom the developer menu (or Ctrl-R) to reload your project without having to restart Tropy. That's not a solution, obviously, but may speed up resetting the image viewer at least. Also, with developer mode enabled, you could turn on the dev-tools console (toggle developer tools via the developer menu): when the issue happens (i.e., image viewer turner staying grey), could you take a look to see if there are any error messages in the console?