Fix lost focus when modals open/close (#12437)

* Fix lost focus after modal closes

Regression caused by the use of the wicg-inert polyfill

* Fix regression introduced by wicg-inert

* Catch errors to please CodeClimate

app/javascript/mastodon/components/modal_root.js

@@ -56,15 +56,21 @@ export default class ModalRoot extends React.PureComponent {
     } else if (!nextProps.children) {
       this.setState({ revealed: false });
     }
-    if (!nextProps.children && !!this.props.children) {
-      this.activeElement.focus();
-      this.activeElement = null;
-    }
   }
 
   componentDidUpdate (prevProps) {
     if (!this.props.children && !!prevProps.children) {
       this.getSiblings().forEach(sibling => sibling.removeAttribute('inert'));
+
+      // Because of the wicg-inert polyfill, the activeElement may not be
+      // immediately selectable, we have to wait for observers to run, as
+      // described in https://github.com/WICG/inert#performance-and-gotchas
+      Promise.resolve().then(() => {
+        this.activeElement.focus();
+        this.activeElement = null;
+      }).catch((error) => {
+        console.error(error);
+      });
     }
     if (this.props.children) {
       requestAnimationFrame(() => {