fix(return): pull up parent's margin only when the return is the last…

… statement.

Fix the issue of rendering:
B A
A.method {
  return x
  B.m
}

src/components/DiagramFrame/SeqDiagram/MessageLayer/Block/Statement/Return/Return.vue

@@ -117,10 +117,8 @@ export default {
 </script>
 
 <style scoped>
-.interaction.return {
-  /* It pulls up the parent's (.statement-container) bottom margin. The purpose is to impact only when return is
-     the last statement. It wouldn't cause an issue if we apply to all return elements, because the following
-     statements will have a margin-top:16px. */
+.statement-container:last-child > .interaction.return {
+  /* It pulls up the parent's (.statement-container) bottom margin. */
   margin-bottom: -16px;
   bottom: -2px; /* The width of the border of the Occurrence */
 }