fix: freeze the scope when the transport is stopped

FL keeps calling process() with silence while stopped/paused, so the scope
scrolled silence instead of holding. Gate the plot's bucket advance on
context.transport().playing, so it freezes on stop/pause and resumes on play.
Bar meters still fall to silence as before. Update README thread-safety notes
for the ScopeRing feed and transport gating.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -256,9 +256,15 @@ NIH-plug provides `Smoother` — use it for all gain/threshold params to avoid z
  
 ### Thread safety
 Params are atomics. The editor and audio thread communicate only through params and a shared
-`Arc<Meters>` of **lock-free atomics** (`meters.rs`) — never a mutex on the audio path. `process()`
-publishes one value per meter per block (gated on the editor being open); the editor reads them
-each frame. Never pass DSP state to the UI directly.
+`Arc<Meters>` (`meters.rs`) — never a mutex on the audio path. Two lock-free feeds, both gated on
+the editor being open:
+- **Bar meters** — decayed atomic scalars, one store per block; the editor reads them each frame.
+- **Scrolling plot** — a single-producer/single-consumer `ScopeRing` of buckets clocked at
+  ~200 Hz, so the plot's horizontal resolution is decoupled from the ~60 fps repaint. The editor
+  drains all new buckets each frame. The scope is **transport-gated** (advances only while playing)
+  so it freezes rather than scrolling silence when the host is stopped/paused.
+
+Never pass DSP state to the UI directly.
  
 ### VST3 licensing
 You must accept Steinberg's VST3 SDK licence before distributing VST3 binaries.

src/lib.rs

@@ -171,7 +171,7 @@ impl Plugin for Codename206 {
         &mut self,
         buffer: &mut Buffer,
         _aux: &mut AuxiliaryBuffers,
-        _context: &mut impl ProcessContext<Self>,
+        context: &mut impl ProcessContext<Self>,
     ) -> ProcessStatus {
         let lookahead = self.lookahead_samples();
 
@@ -198,6 +198,9 @@ impl Plugin for Codename206 {
 
         // Only do the (cheap) metering work when the editor is actually open.
         let metering = self.params.editor_state.is_open();
+        // The host keeps calling process() with silence while stopped/paused (FL does), so the
+        // scope is gated on the transport actually playing — otherwise it would scroll silence.
+        let playing = context.transport().playing;
         let num_samples = buffer.samples();
         let mut lvl_l = [0.0f32; meters::NUM_CHANNELS];
         let mut lvl_r = [0.0f32; meters::NUM_CHANNELS];
@@ -246,11 +249,13 @@ impl Plugin for Codename206 {
                     lvl_l[b] = lvl_l[b].max(out_l);
                     lvl_r[b] = lvl_r[b].max(out_r);
                     gr[b] = gr[b].max(g);
+                    if playing {
                         self.scope_in[b] = self.scope_in[b].max(in_mono);
                         self.scope_out[b] = self.scope_out[b].max(out_l.max(out_r));
                         self.scope_gr[b] = self.scope_gr[b].max(g);
                     }
                 }
+            }
 
             // 'All' aggregate channel over the summed bands (driven before its compressor).
             let all_pre = util::db_to_gain(self.params.all.pre_gain_db.smoothed.next());
@@ -272,6 +277,10 @@ impl Plugin for Codename206 {
                 lvl_r[ALL] = lvl_r[ALL].max(out_r);
                 gr[ALL] = gr[ALL].max(g);
                 lim_gr = lim_gr.max(self.limiter.gain_reduction_db());
+
+                // Only advance the scope while the transport is playing, so it freezes (rather than
+                // scrolling silence) when the host is paused/stopped but still calling process().
+                if playing {
                     self.scope_in[ALL] = self.scope_in[ALL].max(in_mono);
                     self.scope_out[ALL] = self.scope_out[ALL].max(out_l.max(out_r));
                     self.scope_gr[ALL] = self.scope_gr[ALL].max(g);
@@ -286,6 +295,7 @@ impl Plugin for Codename206 {
                         self.scope_samples = 0;
                     }
                 }
+            }
 
             for ch in 0..n {
                 *frame.get_mut(ch).unwrap() = lim_frame[ch];