perf[Navbar]: add scroll bar when the subMenu is too long (#1619)

src/styles/sidebar.scss

@@ -170,8 +170,27 @@
   .nest-menu .el-submenu>.el-submenu__title,
   .el-menu-item {
     &:hover {
-      // You can use $subMenuHover
+      // you can use $subMenuHover
       background-color: $menuHover !important;
     }
   }
+
+  // the scroll bar appears when the subMenu is too long
+  >.el-menu--popup {
+    max-height: 100vh;
+    overflow-y: auto;
+
+    &::-webkit-scrollbar-track-piece {
+      background: #d3dce6;
+    }
+
+    &::-webkit-scrollbar {
+      width: 6px;
+    }
+
+    &::-webkit-scrollbar-thumb {
+      background: #99a9bf;
+      border-radius: 20px;
+    }
+  }
 }