ensure focus does not scroll the modal

In IOS we found that the focus of an element would scroll the page so such element is brought into view.

In our case we have a grid of several views (iframes) and whenever a modal would appear on one of those views then the whole app would scroll to show the cell were the modal appeared, which is undesireable.

This PR fixes that.

src/components/ModalPortal.js

@@ -233,7 +233,7 @@ export default class ModalPortal extends Component {
 
   // Don't steal focus from inner elements
   focusContent = () =>
-    this.content && !this.contentHasFocus() && this.content.focus();
+    this.content && !this.contentHasFocus() && this.content.focus({ preventScroll: true });
 
   closeWithTimeout = () => {
     const closesAt = Date.now() + this.props.closeTimeoutMS;