pref：优化Tree里的箭头层级

@@ -473,14 +473,14 @@
 </script>
 <style lang="less">
   .fold-left {
-    z-index: 9999;
+    z-index: 1;
     cursor: pointer;
     position: absolute;
     top: 0.85rem;
     left: 1.1rem;
   }
   .fold-right {
-    z-index: 9999;
+    z-index: 1;
     cursor: pointer;
     position: absolute;
     top: 0.85rem;