fix: extract closeWS helper, flush pending deltas on close and page unload

templates/document_edit.html

···
             clearTimeout(wsEditTimer);
             pendingDeltas = [];
             stopHeartbeat();
+            ws = null;
             updatePresence([]);
             scheduleReconnect();
         };
 
         ws.onerror = () => {
-            ws.close();
+            closeWS();
         };
     }
 
···
                 ws.send(JSON.stringify({ type: 'ping' }));
                 wsMissedPings++;
                 if (wsMissedPings >= 3) {
-                    ws.close();
+                    closeWS();
                 }
             }
         }, 30000);
···
         pendingDeltas = [];
     }
 
+    function closeWS() {
+        if (!ws) return;
+        clearTimeout(wsEditTimer);
+        flushDeltas();   // send any buffered deltas before closing
+        ws.close();
+        ws = null;
+        clearInterval(wsPingTimer);
+    }
+
     // sendEdit sends a full-document replacement via the granular-delta path.
     // to: -1 is a sentinel meaning "end of document" — the server OT engine
     // clamps it to len(documentText), and applyRemoteEdit clamps it to docLen.
···
         connectWebSocket();
         loadComments();
     }
+
+    window.addEventListener('beforeunload', () => {
+        clearTimeout(wsEditTimer);
+        flushDeltas();
+    });
 </script>
 {{end}}