Dropdown scrollable menu

Sometimes dropdown menu is too long and very useful to limit number of items.

Menu item 1 Menu item 2 Menu item 3 Menu item 4 Menu item 5 Menu item 6 Menu item 7 Menu item 8 Menu item 9 Menu item 10 Menu item 11 Menu item 12 Menu item 13 Menu item 14 Menu item 15 Menu item 16 Menu item 17 Menu item 18 Menu item 19 Menu item 20 
<!-- Source -->
<div class="btn-group dropdown">
  <button class="btn btn-secondary dropdown-toggle" type="button" data-toggle="dropdown">Dropdown</button>
  <div class="dropdown-menu dropdown-menu-scrollable"></div>
</div>