Well, that was interesting.
Fired up IdleMaster and it said I had one game to idle (displayed an ID#, no name) and crashed.
Tried a second time and it ran okay, but I see in my friends list that I'm now showing as in-game for "Steam Summer Sale", which almost immediately gets me a card drop. (This is the first time I've gotten a sale card to drop since the first day.) I hastily killed IdleMaster, though I ended with a second card dropping right after (maybe that woke something up?)
I hope I didn't just commit Steam account suicide without realizing it.