I'm leaning towards your suggestion that the display is popping up somewhere off screen.
Bingo!
Rebooted. No joy.
Borrowed a screen from my client, fired up in "duplicate mode". No Joy.
Changed to Extended Desktop. No Joy.
Re-arranged the position of the screens (from side to side to top/bottom). Hit "apply".
Bingo! There's the editor. On the second screen.
At the moment I'm blaming my display driver. later this week (or early next) I'll see if I can reproduce the problem.
Observation: if I fire up the editor from the manager, I have only one RTT app on the task bar. If I do it via CTRL-click, then I end up with two instances.
(As you suspect, I'm not allowed to play with HKEYs.)