If you run the "Show" command on a target that doesn't have any files that the viewer can show, Dopus loses focus and no viewer window is launched. So far so good (albeit I would prefer if Dopus didn't lose focus), but what also happens is that Dopus triggers the "Working In Background" mouse pointer, and that doesn't go away until you've successfully launched and closed the Viewer. Sometimes it doesn't go away until you close and reopen the whole Lister.
I don't know if this is just a visual bug, or if there actually is a leftover thread running somewhere, but I guess you can find out and prioritise it accordingly 
This mouse pointer:
Dopus: 11.5.1 (Beta) x64
OS: Windows 8.1 Pro