add loading gif

css/style.css

@@ -20,6 +20,7 @@ ul {
 
 ul:nth-child(1) {
   padding: 1em 1em;
+  position: absolute;
 }
 
 li:hover {
@@ -39,7 +40,7 @@ li::before {
 }
 
 .caret-down::before {
-  transform: rotate(90deg);  
+  transform: rotate(90deg);
 }
 .nested {
   display: none;
@@ -72,6 +73,7 @@ aside {
   grid-area: aside;
   overflow-y: scroll;
   margin-right: 2px solid #c3c3c3;
+  position: relative;
 }
 
 footer {
@@ -84,3 +86,14 @@ footer {
 sub {
   display: block;
 }
+
+.loading {
+  position: absolute;
+  z-index: 5;
+  display: none;
+  background-color: rgba(0, 0, 0, 0.438);
+  width: 100%;
+  height: 100%;
+  justify-items: center;
+  align-items: center;
+}