Apply the shape test to a mixed track too

A track that flags some of its own display sets and not others proves the
authoring house makes the distinction, so it needs no sibling to
corroborate the flag being in use -- but it still has to look like a full
dialogue track before a forced label is cleared. A small track with a
couple of flagged signs is a forced track, and demoting it is the mistake
the shape test exists to prevent.

src/mux/codec/pgs.rs

@@ -130,13 +130,18 @@ pub const DEMOTE_MIN_DISPLAY_SHARE_DIVISOR: u32 = 4;
 /// track, and demoting on it would strip a correct forced label from every
 /// track on the disc.
 ///
-/// So the rule is:
+/// So the rule is, in order:
-///   * a track that itself mixes forced and non-forced sets is self-evidently
+///   * something must have been observed at all;
-///     not a forced-only track — demote, no further evidence needed; otherwise
+///   * the flag must be IN USE — on some other track (`disc_uses_forced_flag`) or
-///   * some OTHER track must demonstrably use the flag (`disc_uses_forced_flag`),
+///     on this very track, which is the stronger form: a track carrying the flag
-///     proving the authoring house sets it, AND this track must have the SHAPE of
+///     on some of its sets and not others shows the authoring house making that
-///     a full track ([`DEMOTE_MIN_DISPLAY_SETS`] and
+///     distinction deliberately;
-///     [`DEMOTE_MIN_DISPLAY_SHARE_DIVISOR`]) rather than of a forced-narrative one.
+///   * and the track must have the SHAPE of a full dialogue track
+///     ([`DEMOTE_MIN_DISPLAY_SETS`] and [`DEMOTE_MIN_DISPLAY_SHARE_DIVISOR`])
+///     rather than of a forced-narrative one. This applies to the mixed case too:
+///     a SMALL track with a couple of flagged sets is a forced track whose
+///     authoring flagged some of its signs, and demoting it would be exactly the
+///     mistake the shape test exists to prevent.
 ///
 /// `busiest_displays` is the largest `displays` over every subtitle track judged
 /// together (the same title's tracks for the probe, the same file's tracks for
@@ -145,12 +150,8 @@ pub fn demotable(facts: ForcedFacts, disc_uses_forced_flag: bool, busiest_displa
     if facts.displays == 0 {
         return false;
     }
-    // Mixed: forced sets AND non-forced sets on the same track. The flag is in
+    let flag_in_use = disc_uses_forced_flag || facts.forced_displays > 0;
-    // use right here, so its absence on the other sets is real evidence.
+    if !flag_in_use || facts.displays < DEMOTE_MIN_DISPLAY_SETS {
-    if facts.forced_displays > 0 && facts.forced_displays < facts.displays {
-        return true;
-    }
-    if !disc_uses_forced_flag || facts.displays < DEMOTE_MIN_DISPLAY_SETS {
         return false;
     }
     // `displays >= busiest / DIVISOR`, multiplied out (u64: `displays` is a
@@ -835,11 +836,24 @@ mod tests {
     }
 
     /// A track that itself mixes forced and non-forced display sets needs no
-    /// corroboration: the flag is demonstrably in use ON THIS TRACK, so it is a
+    /// corroboration from a sibling: the flag is demonstrably in use ON THIS
-    /// full track carrying occasional forced signs — not a forced-only track.
+    /// TRACK. Measured shape this models: a busy track labelled forced that
+    /// flags one or two of its hundred-odd display sets.
     #[test]
-    fn a_mixed_track_is_demotable_on_its_own_evidence() {
+    fn a_mixed_track_corroborates_the_flag_itself() {
-        assert!(demotable(facts(4, 1), false, 4));
+        assert!(demotable(facts(108, 2), false, 137));
+    }
+
+    /// ...but the shape test still applies to it. A SMALL track with a couple of
+    /// flagged sets is a forced track whose authoring flagged some of its signs —
+    /// demoting that is the exact mistake the shape test exists to prevent.
+    #[test]
+    fn a_small_mixed_track_is_not_demotable_against_a_busy_disc() {
+        assert!(!demotable(facts(30, 1), true, 2_000));
+        assert!(
+            !demotable(facts(4, 1), true, 4),
+            "and too few sets to say anything either way"
+        );
     }
 
     /// With the flag in use elsewhere on the disc, the shape decides. Measured: