1 files changed, 39 insertions, 0 deletions
diff --git a/app/assets/javascripts/lib/utils/sticky.js b/app/assets/javascripts/lib/utils/sticky.js
new file mode 100644
index 00000000000..098afcfa1b4
--- /dev/null
+++ b/app/assets/javascripts/lib/utils/sticky.js
@@ -0,0 +1,39 @@
+export const createPlaceholder = () => {
+  const placeholder = document.createElement('div');
+  placeholder.classList.add('sticky-placeholder');
+
+  return placeholder;
+};
+
+export const isSticky = (el, scrollY, stickyTop, insertPlaceholder) => {
+  const top = Math.floor(el.offsetTop - scrollY);
+
+  if (top <= stickyTop && !el.classList.contains('is-stuck')) {
+    const placeholder = insertPlaceholder ? createPlaceholder() : null;
+    const heightBefore = el.offsetHeight;
+
+    el.classList.add('is-stuck');
+
+    if (insertPlaceholder) {
+      el.parentNode.insertBefore(placeholder, el.nextElementSibling);
+
+      placeholder.style.height = `${heightBefore - el.offsetHeight}px`;
+    }
+  } else if (top > stickyTop && el.classList.contains('is-stuck')) {
+    el.classList.remove('is-stuck');
+
+    if (insertPlaceholder && el.nextElementSibling && el.nextElementSibling.classList.contains('sticky-placeholder')) {
+      el.nextElementSibling.remove();
+    }
+  }
+};
+
+export default (el, stickyTop, insertPlaceholder = true) => {
+  if (!el) return;
+
+  if (typeof CSS === 'undefined' || !(CSS.supports('(position: -webkit-sticky) or (position: sticky)'))) return;
+
+  document.addEventListener('scroll', () => isSticky(el, window.scrollY, stickyTop, insertPlaceholder), {
+    passive: true,
+  });
+};