Bug 1558312 - Optimistic loading for DS images (#5111)

content-src/components/DiscoveryStreamComponents/DSImage/DSImage.jsx

@@ -42,7 +42,13 @@ export class DSImage extends React.PureComponent {
   }
 
   componentDidMount() {
-    this.observer = new IntersectionObserver(this.onSeen.bind(this));
+    this.observer = new IntersectionObserver(this.onSeen.bind(this), {
+      // Assume an image will be eventually seen if it is within 520px of the viewport
+      // This is half the average Desktop vertical screen size:
+      // http://gs.statcounter.com/screen-resolution-stats/desktop/north-america
+      rootMargin: `540px`,
+    });
+
     this.observer.observe(ReactDOM.findDOMNode(this));
   }