add scrollIntoView handler

Closes #40

https://github.com/reactioncommerce/reaction-docs/pull/686/files

facebook/docusaurus#823

Author: alexkrolick

website/siteConfig.js

@@ -109,6 +109,7 @@ const siteConfig = {
     'https://buttons.github.io/buttons.js',
     'https://cdnjs.cloudflare.com/ajax/libs/clipboard.js/2.0.0/clipboard.min.js',
     '/js/code-block-buttons.js',
+    '/js/sidenav.js',
   ],
 
   stylesheets: [

website/static/js/sidenav.js

@@ -0,0 +1,23 @@
+// See https://github.com/reactioncommerce/reaction-docs/pull/686/files
+// https://github.com/facebook/Docusaurus/issues/823
+
+document.addEventListener('DOMContentLoaded', () => {
+  // Find the active nav item in the sidebar
+  const item = document.getElementsByClassName('navListItemActive')[0]
+  if (!item) {
+    return
+  }
+  const bounding = item.getBoundingClientRect()
+  if (
+    bounding.top >= 0 &&
+    bounding.bottom <=
+      (window.innerHeight || document.documentElement.clientHeight)
+  ) {
+    // Already visible.  Do nothing.
+  } else {
+    // Not visible.  Scroll sidebar.
+    item.scrollIntoView({ block: 'center', inline: 'nearest' })
+    document.body.scrollTop = 0
+    document.documentElement.scrollTop = 0
+  }
+})