Revert "remove dead code"

This reverts commit 267b9e05.

src/App.scss

@@ -553,6 +553,23 @@ nav {
   color: var(--faint, $fallback--faint);
   box-shadow: 0px 0px 4px rgba(0,0,0,.6);
   box-shadow: var(--topBarShadow);
+
+  .back-button {
+    display: block;
+    max-width: 99px;
+    transition-property: opacity, max-width;
+    transition-duration: 300ms;
+    transition-timing-function: ease-out;
+
+    i {
+      margin: 0 1em;
+    }
+
+    &.hidden {
+      opacity: 0;
+      max-width: 5px;
+    }
+  }
 }
 
 .fade-enter-active, .fade-leave-active {