I’d agree that going with the preference setting Peter mentions is the way to go, if this type of popup annoys you at all. End of story. Stop reading now… Unless you don’t want to enable that preference setting (e.g., you still want to be warned if multiple files are to be saved).
The feature request that Peter cites solves the problem specifically asked for:
not to do the popup when only the single viewed file is modified
So the real and exact answer is to make sure you are running 8.4.3 or later.
However, note that the popup will still occur if you invoke Save All and there is exactly one modified file and that file is not the currently active file. The only way it doesn’t occur is if the Save All is invoked and the current file is the only one modified. In this case the Save All is exactly equivalent to a Save, so no popup happens.