Add style for non-overlapping scrollbar in IE (#24081)

scss/_reboot.scss

@@ -239,6 +239,9 @@ pre {
   margin-bottom: 1rem;
   // Don't allow content to break outside
   overflow: auto;
+  // We have @viewport set which causes scrollbars to overlap content in IE11 and Edge, so
+  // we force a non-overlapping, non-auto-hiding scrollbar to counteract.
+  -ms-overflow-style: scrollbar;
 }