The "Ignore track duration difference under this number of seconds" option ignore_track_duration_difference_under, Options > Matching) is meant to suppress the ~length difference indication in the Metadata box when the file and MusicBrainz lengths differ by  less than the configured tolerance. It no longer has any visible effect: the length difference is always highlighted regardless of the setting.

Steps to reproduce
  
1. Set Options > Matching > "Ignore track duration difference under this number of seconds" to e.g. 2.
2. Load a file whose actual duration differs from the matched MusicBrainz recording by less than the tolerance (e.g. 1 second, typically due to rounding).
3. Match/save the file and look at the Length row in the Metadata box.
  
Expected: No difference is highlighted for ~length when the difference is under the tolerance.
  
Actual: The length difference is highlighted (char-level diff shown) regardless of the tolerance value.

Also the current minimal value for the option is 1, it would be convenient to be able to set it to 0 to always be able to see differences.

Note (out of scope):
The fact this option is under Advanced > Matching is very misleading, since it has no impact on matching (and never had).
This option arguably belongs under a Metadata-box display page (perhaps Interface > Metadata Box, where we could also move current Top tags).
This ticket is only about restoring the tolerance's effect on the ~length diff display.