Add fade animation to sidebar popup

jet/static/jet/css/_sidebar.scss

@@ -203,33 +203,27 @@
   &-popup {
     position: absolute;
     top: 0;
-    left: $sidebar-width;
     bottom: 0;
     width: $sidebar-width;
     color: $sidebar-popup-text-color;
     background-color: $sidebar-popup-background-color;
-    z-index: 3;
     overflow-y: auto;
 
     &-container {
       display: none;
-    }
-
-    &-item {
-      display: none;
-    }
-
-    &-background {
+      background-color: transparentize($sidebar-popup-overlay-color, 0.5);
       position: absolute;
       top: 0;
       left: $sidebar-width;
       bottom: 0;
       right: 0;
-      background-color: $sidebar-popup-overlay-color;
-      opacity: 0.5;
       z-index: 2;
     }
 
+    &-item {
+      display: none;
+    }
+
     &-title {
       font-size: 12px;
       font-weight: bold;

jet/templates/admin/base.html

@@ -276,7 +276,6 @@
                             </div>
                         {% endfor %}
                     </div>
-                    <div class="sidebar-popup-background"></div>
                 </div>
             {% endif %}
             </div>