Dynamically create offset for sticky bar

app/assets/javascripts/lib/utils/sticky.js

-export const isSticky = (el, scrollY, stickyTop) => {
+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) {
+  if (top <= stickyTop && !el.classList.contains('is-stuck')) {
+    const placeholder = insertPlaceholder ? createPlaceholder(el) : null;
+    const heightBefore = el.offsetHeight;
+
     el.classList.add('is-stuck');
-  } else {
+
+    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.classList.contains('sticky-placeholder')) {
+      el.nextElementSibling.remove();
+    }
   }
 };
-export default (el) => {
+export default (el, insertPlaceholder = true) => {
   if (!el) return;
   const computedStyle = window.getComputedStyle(el);
@@ -17,7 +37,7 @@ export default (el) => {
   const stickyTop = parseInt(computedStyle.top, 10);
-  document.addEventListener('scroll', () => isSticky(el, window.scrollY, stickyTop), {
+  document.addEventListener('scroll', () => isSticky(el, window.scrollY, stickyTop, insertPlaceholder), {
     passive: true,
   });
 };

app/assets/stylesheets/pages/diff.scss

@@ -451,7 +451,7 @@
 }
 .files {
-  margin-top: -1px;
+  margin-top: 1px;
   .diff-file:last-child {
     margin-bottom: 0;
@@ -586,11 +586,6 @@
       top: 76px;
     }
-    + .files,
-    + .alert {
-      margin-top: 1px;
-    }
-
     &:not(.is-stuck) .diff-stats-additions-deletions-collapsed {
       display: none;
     }
@@ -608,7 +603,7 @@
       + .files,
       + .alert {
-        margin-top: 32px;
+        // margin-top: 32px;
       }
     }
   }