fix: Do not increment _recording_frame_num on each elem; instead, on …

…each step

bindings/pydrake/systems/meshcat_visualizer.py

@@ -382,14 +382,16 @@ def DoPublish(self, context, event):
             # The MBP parsers only register the plant as a nameless source.
             # TODO(russt): Use a more textual naming convention here?
             pose_matrix = pose_bundle.get_transform(frame_i)
-            self.vis[self.prefix][source_name][str(model_id)][frame_name]\
-                .set_transform(pose_matrix.GetAsMatrix4())
+            cur_vis = (
+                self.vis[self.prefix][source_name][str(model_id)][frame_name])
+            cur_vis.set_transform(pose_matrix.GetAsMatrix4())
             if self._is_recording:
                 with self._animation.at_frame(
-                    self.vis[self.prefix][source_name][str(model_id)]
-                        [frame_name], self._recording_frame_num) as frame:
-                    frame.set_transform(pose_matrix.GetAsMatrix4())
-                self._recording_frame_num += 1
+                        cur_vis, self._recording_frame_num) as cur_vis_frame:
+                    cur_vis_frame.set_transform(pose_matrix.GetAsMatrix4())
+
+        if self._is_recording:
+            self._recording_frame_num += 1
 
     def start_recording(self):
         """