Initial attempt at accessibly announcing route updates.

* Add handler for `phx:page-loading-stop` that focuses either the first `<h1>` child of `<main>`, or `<main>` directly if no child `<h1>` is present. Make this our focus target.
* Cache original `tabindex` of target, if any, and assign it a temporary `tabindex1 of -1.
* Focus the target.
* After a long timeout, either restore the target's original `tabindex` or remove the temporary -1 value. Short timeouts didn't seem to perform the focus step. Neither did nested `requestAnimationFrame` calls.

assets/js/app.js

@@ -177,11 +177,30 @@ let liveSocket = new LiveSocket("/live", Socket, {
   params: {_csrf_token: csrfToken}
 })
 
+let routeUpdated = () => {
+  let target = document.querySelector("main h1") || document.querySelector("main")
+  if (target) {
+    let origTabIndex = target.getAttribute("tabindex")
+    target.setAttribute("tabindex", "-1")
+    target.focus()
+    window.setTimeout(() => {
+      if (origTabIndex) {
+        target.setAttribute("tabindex", origTabIndex)
+      } else {
+        target.removeAttribute("tabindex")
+      }
+    }, 1000)
+  }
+}
+
 // Show progress bar on live navigation and form submits
 topbar.config({barColors: {0: "#29d"}, shadowColor: "rgba(0, 0, 0, .3)"})
 window.addEventListener("phx:page-loading-start", info => topbar.show())
 window.addEventListener("phx:page-loading-stop", info => topbar.hide())
 
+// Accessible routing
+window.addEventListener("phx:page-loading-stop", () => window.requestAnimationFrame(routeUpdated))
+
 window.addEventListener("js:exec", e => e.target[e.detail.call](...e.detail.args))
 
 // connect if there are any LiveViews on the page