fix(demo): fix demo header static position for safari browser (#3600)

close #3312

Author: artolshansky

demo/src/assets/css/style.scss

@@ -517,14 +517,17 @@ a:hover {
 /* above 768px */
 @media (min-width: 768px) {
   .header {
-    position: -webkit-sticky;
-    position: sticky;
+    position: fixed;
+    width: 100%;
     top: 0;
     z-index: 1090; // to overlap most positioned elements in bootstrap
   }
   .header-dropdown {
     z-index: 1090;
   }
+  .main {
+    padding-top: $header-height;
+  }
   .modal-backdrop {
     z-index: 2000; // to overlap header :)
   }