Keep merging fader gestures once the undo history is full

Found while testing crates/engine/src/lib.rs:368-381 (Engine::execute_merged, with push_undo at 280-291)

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/engine/src/lib.rs

@@ -369,7 +369,15 @@ impl Engine {
         let key = self.merge_hold.clone().unwrap_or_else(|| key.to_string());
         let continuing = self.merge_key.as_ref() == Some(&key);
         let before_len = self.undo.len();
-        let r = self.execute(id, params)?;
+        // The step pushed for a continuing tick is dropped again below, so it must not trim the
+        // oldest entry first when the history is full.
+        let limit = self.history_limit;
+        if continuing && before_len > 0 {
+            self.history_limit = limit.max(before_len.saturating_add(1));
+        }
+        let r = self.execute(id, params);
+        self.history_limit = limit;
+        let r = r?;
         if continuing && self.undo.len() == before_len + 1 && before_len > 0 {
             // Drop the step just pushed; the earlier one still holds the pre-gesture document.
             self.undo.pop();

crates/engine/tests/fader_merge_full_history.rs

@@ -0,0 +1,42 @@
+use serde_json::json;
+use soundcraft_engine::Engine;
+use soundcraft_model::{ChannelFormat, TrackKind};
+
+fn engine_with_full_history(limit: usize) -> (Engine, u64) {
+    let mut e = Engine::default();
+    let t = e.session_mut().add_track(TrackKind::Audio, ChannelFormat::Mono, None).0;
+    e.history_limit = limit;
+    for i in 0..limit + 1 {
+        e.execute("mix.pan", &json!({"track": t, "pan": 0.1 * (i as f64 + 1.0)})).unwrap();
+    }
+    assert_eq!(e.undo_history().len(), limit);
+    (e, t)
+}
+
+#[test]
+fn a_fader_gesture_is_one_undo_step_when_the_history_is_full() {
+    let (mut e, t) = engine_with_full_history(3);
+    for db in [-1.0, -2.0, -3.0, -4.0, -5.0] {
+        e.execute_merged("mix.volume", &json!({"track": t, "db": db}), "fader").unwrap();
+    }
+    e.end_merge();
+    let volume = |e: &Engine| e.session().tracks.iter().find(|x| x.id.0 == t).map(|x| x.mixer.volume_db);
+    assert_eq!(volume(&e), Some(-5.0));
+    // The gesture costs one step; it must not push further real edits out of the history.
+    assert_eq!(e.undo_history().len(), 3, "history: {:?}", e.undo_history());
+    assert_eq!(e.undo_history().iter().filter(|l| l.as_str() == "Set Volume").count(), 1, "history: {:?}", e.undo_history());
+    assert!(e.undo());
+    assert_eq!(volume(&e), Some(0.0), "one undo should undo the whole fader gesture; history: {:?}", e.undo_history());
+}
+
+#[test]
+fn a_zero_history_limit_keeps_no_undo_entry_for_a_gesture() {
+    let mut e = Engine::default();
+    let t = e.session_mut().add_track(TrackKind::Audio, ChannelFormat::Mono, None).0;
+    e.history_limit = 0;
+    for db in [-1.0, -2.0, -3.0] {
+        e.execute_merged("mix.volume", &json!({"track": t, "db": db}), "fader").unwrap();
+    }
+    e.end_merge();
+    assert!(e.undo_history().is_empty(), "history: {:?}", e.undo_history());
+}