fix: prevent mouse event when shouldCloseOnOverlayClick = false.

This is tricky, if the user don't allow close when clicking on the
overlay, we need to stop the event so the focus won't leave the modal
(where the key handler is defined).

closes reactjs#186.

examples/basic/simple_usage/index.js

@@ -76,6 +76,7 @@ class SimpleUsage extends Component {
           closeTimeoutMS={150}
           contentLabel="modalB"
           isOpen={currentModal == MODAL_B}
+          shouldCloseOnOverlayClick={false}
           onAfterOpen={this.handleOnAfterOpenModal}
           onRequestClose={this.toggleModal(MODAL_B)}>
           <h1 id="heading" ref={h1 => this.heading = h1}>This is the modal 2!</h1>

src/components/ModalPortal.js

@@ -242,7 +242,10 @@ export default class ModalPortal extends Component {
     this.shouldClose = false;
   };
 
-  handleOverlayOnMouseDown = () => {
+  handleOverlayOnMouseDown = event => {
+    if (!this.props.shouldCloseOnOverlayClick) {
+      event.preventDefault();
+    }
     this.moveFromContentToOverlay = false;
   };