When opening the dropdown, focus() the dropdown-toggle BEFORE triggering 'shown.bs.dropdown'

Closes #11281 by merging it (after rebasing & resolving conflicts)

js/dropdown.js

@@ -40,11 +40,11 @@
 
       if (e.isDefaultPrevented()) return
 
+      $this.trigger('focus')
+
       $parent
         .toggleClass('open')
         .trigger('shown.bs.dropdown', relatedTarget)
-
-      $this.trigger('focus')
     }
 
     return false