Add fallback behaviour

js/src/dropdown.js

@@ -298,6 +298,12 @@ class Dropdown extends BaseComponent {
           altBoundary: this._config.flip,
           rootBoundary: this._config.boundary
         }
+      },
+      {
+        name: 'flip',
+        options: {
+          fallbackPlacements: ['top', 'right', 'bottom', 'left']
+        }
       }]
     }