I think I can recreate this consistently (well, twice!)
Login to Skydemon, open Document browser, maximised.
Toggle to other programmes, and stay away for long enough.
When I ALT-TAB back to Skydemon both the Document Browser and Skydemon appear as separate options in the ALT-TAB mosaic.
If I select Skydemon - that's fine, 'I'm still here' is accessible, and I can carry on.
If however I select Document Browser - it's just a frozen document browser window, no 'I'm still here' box. Subsequent attempts to then ALT-TAB to the main Skydemon window don't work - just returning me back to the frozen Document Browser window.
The slightly bizarre bit, is if I then open any other random programme (e.g. explorer, or command prompt) and then close the other random programme - the 'I'm still here' box then reappears.