Optimize layout and stylesheet

src/assets/css/components/common/sidebar.scss

@@ -100,10 +100,21 @@
           }
 
           .submenu {
-            display: none;
+            visibility: hidden;
             position: fixed;
             padding-left: 20px;
             z-index: 10000;
+            animation: 0.1s ease forwards;
+            @include mixins.unique-keyframes {
+              0% {
+                transform: translateX(0);
+                opacity: 1;
+              }
+              100% {
+                transform: translateX(-10px);
+                opacity: 0;
+              }
+            }
 
             .content {
               display: flex;
@@ -144,7 +155,7 @@
             }
 
             .submenu {
-              display: block;
+              visibility: visible;
               animation: 0.3s ease;
               @include mixins.unique-keyframes {
                 0% {
@@ -185,6 +196,7 @@
       display: flex;
       justify-content: center;
       align-items: center;
+      flex: 0 0 auto;
       margin-left: 4px;
       width: 36px;
       height: 36px;