add preventDefault

src/client/js/services/EditorContainer.js

@@ -151,7 +151,10 @@ export default class EditorContainer extends Container {
     return opt;
   }
 
+  // See https://developer.mozilla.org/en-US/docs/Web/API/WindowEventHandlers/onbeforeunload#example
   showUnsavedWarning(e) {
+    // Cancel the event
+    e.preventDefault();
     // display browser default message
     e.returnValue = '';
     return '';