better a11y for modal: retain focus when closed

routes/_components/ModalDialog.html

@@ -0,0 +1,87 @@
+<div class="dialog-wrapper" ref:node>
+  <div class="close-dialog-button-wrapper">
+    <button class="close-dialog-button" aria-label="Close dialog" on:click="onCloseButtonClicked()">
+      <span aria-hidden="true">&times;</span>
+    </button>
+  </div>
+  <slot></slot>
+</div>
+<style>
+  :global(.modal-dialog) {
+    position: fixed;
+    top: 50%;
+    transform: translate(0, -50%);
+    background: #000;
+    padding: 0;
+    border: 3px solid var(--main-border);
+  }
+  :global(.modal-dialog-wrapper) {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    max-width: calc(100vw - 40px);
+  }
+  .close-dialog-button-wrapper {
+    text-align: right;
+    width: 100%;
+    background: var(--nav-bg)
+  }
+  .close-dialog-button {
+    margin: 0 0 2px;
+    padding: 0;
+    background: none;
+    border: none;
+  }
+  .close-dialog-button span {
+    padding: 0 15px;
+    font-size: 48px;
+    color: var(--button-primary-text);
+  }
+  :global(dialog.modal-dialog::backdrop) {
+    background: rgba(51, 51, 51, 0.8);
+  }
+  :global(dialog.modal-dialog + .backdrop) {
+    background: rgba(51, 51, 51, 0.8);
+  }
+</style>
+<script>
+
+  import { importDialogPolyfill } from '../_utils/asyncModules'
+  import { registerFocusRestoreDialog } from '../_utils/dialogs'
+
+  export default {
+    oncreate() {
+      // TODO: this hack is for Edge 16, which makes the modal too wide
+      if (typeof setImmediate === 'function' && navigator.userAgent.match(/Edge/)) {
+        this.getDialogElement().style.width = `${this.get('width')}px`
+      }
+      this.observe('shown', shown => {
+        if (shown) {
+          this.show()
+        }
+      })
+      this.registration = this.register()
+    },
+    methods: {
+      async register() {
+        if (typeof HTMLDialogElement === 'undefined') {
+          let dialogPolyfill = await importDialogPolyfill()
+          dialogPolyfill.registerDialog(this.getDialogElement())
+        }
+        registerFocusRestoreDialog(this.getDialogElement())
+      },
+      async show() {
+        await this.registration
+        this.getDialogElement().showModal()
+      },
+      onCloseButtonClicked() {
+        this.getDialogElement().close()
+        document.body.removeChild(this.getDialogElement())
+      },
+      getDialogElement() {
+        return this.refs.node.parentElement
+      }
+    }
+  }
+</script>

routes/_components/status/VideoDialog.html

@@ -1,9 +1,4 @@
-<div class="video-dialog-wrapper">
-  <div class="close-video-button-wrapper">
-    <button class="close-video-button" aria-label="Close dialog" on:click="onCloseButtonClicked()">
-      <span aria-hidden="true">&times;</span>
-    </button>
-  </div>
+<ModalDialog :shown>
   <video poster="{{poster}}"
          src="{{src}}"
          width="{{width}}"
@@ -11,75 +6,22 @@
          aria-label="Video: {{description || ''}}"
          controls
   />
-</div>
+</ModalDialog>
 <style>
-  :global(.video-dialog) {
-    position: fixed;
-    top: 50%;
-    transform: translate(0, -50%);
-    background: #000;
-    padding: 0;
-    border: 3px solid var(--main-border);
-  }
-  :global(.video-dialog-wrapper) {
-    display: flex;
-    flex-direction: column;
-    align-items: center;
-    justify-content: center;
-    max-width: calc(100vw - 40px);
-  }
-  :global(.video-dialog video) {
+  :global(.modal-dialog video) {
     max-width: 100%;
   }
-  .close-video-button-wrapper {
-    text-align: right;
-    width: 100%;
-    background: var(--nav-bg)
-  }
-  .close-video-button {
-    margin: 0 0 2px;
-    padding: 0;
-    background: none;
-    border: none;
-  }
-  .close-video-button span {
-    padding: 0 15px;
-    font-size: 48px;
-    color: var(--button-primary-text);
-  }
-  :global(dialog.video-dialog::backdrop) {
-    background: rgba(51, 51, 51, 0.8);
-  }
-  :global(dialog.video-dialog + .backdrop) {
-    background: rgba(51, 51, 51, 0.8);
-  }
 </style>
 <script>
-
-  import { importDialogPolyfill } from '../../_utils/asyncModules'
+  import ModalDialog from '../ModalDialog.html'
 
   export default {
-    oncreate() {
-      // TODO: this hack is for Edge 16, which makes the modal too wide
-      if (typeof setImmediate === 'function' && navigator.userAgent.match(/Edge/)) {
-        this.get('dialog').style.width = `${this.get('width')}px`
-      }
-      this.registration = this.register()
+    components: {
+      ModalDialog
     },
     methods: {
-      async register() {
-        if (typeof HTMLDialogElement === 'undefined') {
-          let dialogPolyfill = await importDialogPolyfill()
-          dialogPolyfill.registerDialog(this.get('dialog'))
-        }
-      },
-      async showModal() {
-        await this.registration
-        this.get('dialog').showModal()
-      },
-      onCloseButtonClicked() {
-        this.get('dialog').close()
-        document.body.removeChild(this.get('dialog'))
+      async show() {
+        this.set({shown: true})
       }
     }
   }

routes/_utils/dialogs.js

@@ -0,0 +1,55 @@
+// From https://gist.github.com/samthor/babe9fad4a65625b301ba482dad284d1
+// Via https://github.com/GoogleChrome/dialog-polyfill/issues/139
+
+let registered = new WeakMap()
+
+// store previous focused node centrally
+let previousFocus = null
+document.addEventListener('focusout', (e) => {
+  previousFocus = e.target
+}, true)
+
+/**
+ * Updates the passed dialog to retain focus and restore it when the dialog is closed.
+ * @param {!HTMLDialogElement} dialog to upgrade
+ */
+export function registerFocusRestoreDialog(dialog) {
+  // replace showModal method directly, to save focus
+  let realShowModal = dialog.showModal
+  dialog.showModal = function () {
+    let savedFocus = document.activeElement
+    if (savedFocus === document || savedFocus === document.body) {
+      // some browsers read activeElement as body
+      savedFocus = previousFocus
+    }
+    registered.set(dialog, savedFocus)
+    realShowModal.call(this)
+  }
+
+  // on close, try to focus saved, if possible
+  dialog.addEventListener('close', function () {
+    if (dialog.hasAttribute('open')) {
+      return  // in native, this fires the frame later
+    }
+    let savedFocus = registered.get(dialog)
+    if (document.contains(savedFocus)) {
+      let wasFocus = document.activeElement
+      savedFocus.focus()
+      if (document.activeElement !== savedFocus) {
+        wasFocus.focus()  // restore focus, we couldn't focus saved
+      }
+    }
+    savedFocus = null
+  })
+}
+
+export function createDialogElement(label) {
+  if (!label) {
+    throw new Error('the modal must have a label')
+  }
+  let dialogElement = document.createElement('dialog')
+  dialogElement.classList.add('modal-dialog')
+  dialogElement.setAttribute('aria-label', label)
+  document.body.appendChild(dialogElement)
+  return dialogElement
+}

routes/_utils/showVideoDialog.js

@@ -1,20 +1,16 @@
 import VideoDialog from '../_components/status/VideoDialog.html'
+import { createDialogElement } from './dialogs'
 
 export function showVideoDialog(poster, src, width, height, description) {
-  let dialog = document.createElement('dialog')
-  dialog.classList.add('video-dialog')
-  dialog.setAttribute('aria-label', 'Video dialog')
-  document.body.appendChild(dialog)
   let videoDialog = new VideoDialog({
-    target: dialog,
+    target: createDialogElement('Video dialog'),
     data: {
       poster: poster,
       src: src,
-      dialog: dialog,
       width: width,
       height: height,
       description: description
     }
   })
-  videoDialog.showModal()
+  videoDialog.show()
 }