1 files changed, 58 insertions, 0 deletions
diff --git a/app/assets/javascripts/lib/utils/resize_observer.js b/app/assets/javascripts/lib/utils/resize_observer.js
new file mode 100644
index 00000000000..e72c6fe1679
--- /dev/null
+++ b/app/assets/javascripts/lib/utils/resize_observer.js
@@ -0,0 +1,58 @@
+import { contentTop } from './common_utils';
+
+const interactionEvents = ['mousedown', 'touchstart', 'keydown', 'wheel'];
+
+export function createResizeObserver() {
+  return new ResizeObserver((entries) => {
+    entries.forEach((entry) => {
+      entry.target.dispatchEvent(new CustomEvent(`ResizeUpdate`, { detail: { entry } }));
+    });
+  });
+}
+
+// watches for change in size of a container element (e.g. for lazy-loaded images)
+// and scroll the target element to the top of the content area
+// stop watching after any user input. So if user opens sidebar or manually
+// scrolls the page we don't hijack their scroll position
+export function scrollToTargetOnResize({
+  target = window.location.hash,
+  container = '#content-body',
+} = {}) {
+  if (!target) return null;
+
+  const ro = createResizeObserver();
+  const containerEl = document.querySelector(container);
+  let interactionListenersAdded = false;
+
+  function keepTargetAtTop() {
+    const anchorEl = document.querySelector(target);
+
+    if (!anchorEl) return;
+
+    const anchorTop = anchorEl.getBoundingClientRect().top + window.scrollY;
+    const top = anchorTop - contentTop();
+    document.documentElement.scrollTo({
+      top,
+    });
+
+    if (!interactionListenersAdded) {
+      interactionEvents.forEach((event) =>
+        // eslint-disable-next-line no-use-before-define
+        document.addEventListener(event, removeListeners),
+      );
+      interactionListenersAdded = true;
+    }
+  }
+
+  function removeListeners() {
+    interactionEvents.forEach((event) => document.removeEventListener(event, removeListeners));
+
+    ro.unobserve(containerEl);
+    containerEl.removeEventListener('ResizeUpdate', keepTargetAtTop);
+  }
+
+  containerEl.addEventListener('ResizeUpdate', keepTargetAtTop);
+
+  ro.observe(containerEl);
+  return ro;
+}