handle drag n drop

packages/z-timeline/lib/main.dart

@@ -83,6 +83,7 @@ class _MainAppState extends State<MainApp> {
             start: start,
             end: end,
             lane: item.lane,
+            hasEnd: item.end != null,
           ),
         );
       }
@@ -118,16 +119,21 @@ class _MainAppState extends State<MainApp> {
     String newGroupId,
     int newLane,
   ) {
-    final duration = entry.end.difference(entry.start);
-    final newEnd = newStart.add(duration);
-
-    emitEvent('entry_moved', {
+    final payload = <String, Object?>{
       'entryId': entry.id,
       'newStart': newStart.toIso8601String(),
-      'newEnd': newEnd.toIso8601String(),
       'newGroupId': newGroupId,
       'newLane': newLane,
-    });
+    };
+
+    if (entry.hasEnd) {
+      final duration = entry.end.difference(entry.start);
+      payload['newEnd'] = newStart.add(duration).toIso8601String();
+    } else {
+      payload['newEnd'] = null;
+    }
+
+    emitEvent('entry_moved', payload);
   }
 
   void _emitContentHeight() {