Use intersectionRatio to determine intersection

Some older browsers do not ship with isIntersecting, while they already
have IntersectionObserver support. We make use of `intersectionRatio`
now to fix the Lazy Loader for those browsers.

app/assets/javascripts/lazy_loader.js

@@ -91,7 +91,9 @@ export default class LazyLoader {
 
   onIntersection = entries => {
     entries.forEach(entry => {
-      if (entry.isIntersecting) {
+      // We are using `intersectionRatio > 0` over `isIntersecting`, as some browsers did not ship the latter
+      // See: https://gitlab.com/gitlab-org/gitlab-ce/issues/54407
+      if (entry.intersectionRatio > 0) {
         this.intersectionObserver.unobserve(entry.target);
         this.lazyImages.push(entry.target);
       }

changelogs/unreleased/54407-fix-limited-intersection-observers.yml

@@ -0,0 +1,5 @@
+---
+title: Fix Image Lazy Loader for some older browsers
+merge_request:
+author:
+type: fixed