A game shows as installed but never downloaded

Mac Steam sometimes marks a Windows game as installed although it downloaded nothing: the size is 0 and Play does nothing. This happens in Steam itself, with or without Superposer.

Superposer checks what is really on disk. Such games appear on the Steam Integration page under Not really installed: quit Steam and click Repair…. Each fix is recorded in Changes to Your Mac and can be undone. If Superposer can't tell which version a game is, it only reports it: in Steam use Properties › Installed Files › Verify integrity, or uninstall and install the game again.

Steam shows STREAM instead of INSTALL? That isn't this problem: the game is installed on another computer signed in to your Steam account, and Steam offers to stream it from there.