[fixed] stacked/nested modals have focus lost in Safari

Fixes reactjs#801

If the `keydown` event within the currently-open, nested modal is propagated, Safari will toggle focus between the parent modal's focusable elements instead.

src/components/ModalPortal.js

@@ -275,6 +275,7 @@ export default class ModalPortal extends Component {
 
   handleKeyDown = event => {
     if (event.keyCode === TAB_KEY) {
+      event.stopPropagation();
       scopeTab(this.content, event);
     }