microsite/docs: Scroll nav sidebar to view for loaded page

microsite/siteConfig.js

@@ -83,6 +83,7 @@ const siteConfig = {
     'https://unpkg.com/[email protected]/dist/medium-zoom.min.js',
     '/js/medium-zoom.js',
     '/js/dismissable-banner.js',
+    '/js/scroll-nav-to-view-in-docs.js',
   ],
 
   // On page navigation for the current documentation page.

microsite/static/js/scroll-nav-to-view-in-docs.js

@@ -0,0 +1,23 @@
+// On backstage.io/docs pages, scroll the Nav sidebar to focus on
+// the page being viewed. Helpful when the Nav is large enough that
+// the selected page is hidden somewhere at bottom.
+// Credits: https://github.com/facebook/docusaurus/issues/823#issuecomment-421152269
+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: 'start', inline: 'nearest' });
+    document.body.scrollTop = document.documentElement.scrollTop = 0;
+  }
+});