Copied the scroll.js file from github/auto-complete-element.

Scrolls to the target list element when navigating.

Author: bnjamin

src/combobox-nav.js

@@ -1,5 +1,7 @@
 /* @flow strict */
 
+import {scrollTo} from './scroll'
+
 export function install(input: HTMLTextAreaElement | HTMLInputElement, list: HTMLElement): void {
   input.addEventListener('compositionstart', trackComposition)
   input.addEventListener('compositionend', trackComposition)
@@ -103,6 +105,7 @@ export function navigate(
     if (target === el) {
       input.setAttribute('aria-activedescendant', target.id)
       target.setAttribute('aria-selected', 'true')
+      scrollTo(list, target)
     } else {
       el.setAttribute('aria-selected', 'false')
     }

src/scroll.js

@@ -0,0 +1,15 @@
+/* @flow strict */
+
+export function scrollTo(container: HTMLElement, target: HTMLElement) {
+  if (!inViewport(container, target)) {
+    container.scrollTop = target.offsetTop
+  }
+}
+
+function inViewport(container: HTMLElement, element: HTMLElement): boolean {
+  const scrollTop = container.scrollTop
+  const containerBottom = scrollTop + container.clientHeight
+  const top = element.offsetTop
+  const bottom = top + element.clientHeight
+  return top >= scrollTop && bottom <= containerBottom
+}