diff --git a/rust/pm-grid/src/main.rs b/rust/pm-grid/src/main.rs
index 2e5c2a9..151daf0 100644
--- a/rust/pm-grid/src/main.rs
+++ b/rust/pm-grid/src/main.rs
@@ -216,6 +216,7 @@ fn build_setlists(user: Vec<SetList>) -> Vec<SetList> {
         })
         .collect();
     v.extend(user);
+    v.retain(|s| !s.items.is_empty()); // never keep an empty set list (would `% 0` in load/next)
     v
 }
 
@@ -946,6 +947,12 @@ impl App {
 
     /// Build `F0 7D <op> <text(ASCII-clamped)> F7`, packetize into 4-byte USB-MIDI events, queue them.
     fn sx_send(&mut self, op: u8, text: &str) {
+        // Bound the TX queue: if the host stopped draining MIDI-IN (e.g. editor closed without a BYE
+        // while sync_armed), drop the message rather than grow the heap forever. The 5 s heartbeat
+        // re-syncs once the host returns.
+        if self.tx_q.len() > 256 {
+            return;
+        }
         let mut f: Vec<u8> = Vec::with_capacity(text.len() + 4);
         f.push(0xF0);
         f.push(0x7D);