Note that this doesn't _100%_ guarantee no unnecessary filter processing:
* As NJ pointed out back when the searchFileList() usage was added, the user might type an extra char & then backspace, causing two filter invocations with the same, "non-stale" query string.
* Experimentation shows that Chromium will actually skip ahead if key event processing gets _really_ behind (like > 1 second).  The DOM text field value gets fast-forwaded to the latest value even while older keyup events are getting handled.  The result is typically 2-3 filter invocations with the latest "non-stale" string instead of just one.

Cases like those would be pretty easy to fix if we ditched Smart Autocomplete.  Short of that though, it'd be hackier (e.g. monitoring Smart Autocomplete's keyIn events to figure out what timeouts it's going to queue up that we'll receive later on) -- doesn't seem worth that level of effort.