Adds native lazy image loading

So no need for IntersectionObservers if supported

app/assets/javascripts/lazy_loader.js

@@ -14,6 +14,10 @@ export default class LazyLoader {
     scrollContainer.addEventListener('load', () => this.register());
   }
 
+  static supportsNativeLazyLoading() {
+    return 'loading' in HTMLImageElement.prototype;
+  }
+
   static supportsIntersectionObserver() {
     return Boolean(window.IntersectionObserver);
   }
@@ -23,7 +27,9 @@ export default class LazyLoader {
       () => {
         const lazyImages = [].slice.call(document.querySelectorAll('.lazy'));
 
-        if (LazyLoader.supportsIntersectionObserver()) {
+        if (LazyLoader.supportsNativeLazyLoading()) {
+          lazyImages.forEach(img => LazyLoader.loadImage(img));
+        } else if (LazyLoader.supportsIntersectionObserver()) {
           if (this.intersectionObserver) {
             lazyImages.forEach(img => this.intersectionObserver.observe(img));
           }
@@ -72,11 +78,14 @@ export default class LazyLoader {
   }
 
   register() {
+    if (!LazyLoader.supportsNativeLazyLoading()) {
       if (LazyLoader.supportsIntersectionObserver()) {
         this.startIntersectionObserver();
       } else {
         this.startLegacyObserver();
       }
+    }
+
     this.startContentObserver();
     this.searchLazyImages();
   }
@@ -148,16 +157,12 @@ export default class LazyLoader {
 
   static loadImage(img) {
     if (img.getAttribute('data-src')) {
+      img.setAttribute('loading', 'lazy');
       let imgUrl = img.getAttribute('data-src');
       // Only adding width + height for avatars for now
       if (imgUrl.indexOf('/avatar/') > -1 && imgUrl.indexOf('?') === -1) {
-        let targetWidth = null;
-        if (img.getAttribute('width')) {
-          targetWidth = img.getAttribute('width');
-        } else {
-          targetWidth = img.width;
-        }
-        if (targetWidth) imgUrl += `?width=${targetWidth}`;
+        const targetWidth = img.getAttribute('width') || img.width;
+        imgUrl += `?width=${targetWidth}`;
       }
       img.setAttribute('src', imgUrl);
       img.removeAttribute('data-src');

app/views/layouts/_img_loader.html.haml

+= javascript_tag nonce: true do
+  :plain
+    if ('loading' in HTMLImageElement.prototype) {
+      document.querySelectorAll('img.lazy').forEach(img => {
+        img.loading = 'lazy';
+        let imgUrl = img.dataset.src;
+        // Only adding width + height for avatars for now
+        if (imgUrl.indexOf('/avatar/') > -1 && imgUrl.indexOf('?') === -1) {
+          const targetWidth = img.getAttribute('width') || img.width;
+          imgUrl += `?width=${targetWidth}`;
+        }
+        img.src = imgUrl;
+        img.removeAttribute('data-src');
+        img.classList.remove('lazy');
+        img.classList.add('js-lazy-loaded', 'qa-js-lazy-loaded');
+      });
+    }

app/views/layouts/application.html.haml

@@ -13,4 +13,6 @@
     = render 'layouts/page', sidebar: sidebar, nav: nav
     = footer_message
 
+    = render 'layouts/img_loader'
+
     = yield :scripts_body