On Windows, @atom/watcher uses the ReadDirectoryChangesW() call to monitor a directory for changes. ReadDirectoryChangesW is called in asynchronous mode with a completion routine.
Out-of-band command signalling is handled by an asynchronous procedure call scheduled with QueueUserAPC().
The FILE_NOTIFY_INFORMATION structure provided to the completion routine does not indicate whether the entry is a directory or a file. @atom/watcher uses GetFileAttributesW to identify entry kinds, but leaves them as "unknown" for deletions.
If the filesystem does not support events and ERROR_INVALID_FUNCTION is returned, @atom/watcher will fall back to polling. However, depending on the filesystem, Windows may simply fail to deliver events.
A Windows process can open a maximum of 16,711,680 handles, so the number of watch roots is somewhat less than that 😉