[A11y] Widget: pass doc title in overlay to iframe.title (#5210)

src/pretix/static/pretixpresale/js/ui/iframe.js

@@ -7,4 +7,12 @@ var inIframe = function () {
 };
 if (inIframe()) {
     document.documentElement.classList.add('in-iframe');
+    try {
+        window.parent.postMessage({
+            type: "pretix:widget:title",
+            title: document.title,
+        }, "*");
+    } catch (e) {
+        console.error("Could not post message to parent.", e);
+    }
 }

src/pretix/static/pretixpresale/js/widget/widget.js

@@ -939,7 +939,18 @@ Vue.component('pretix-overlay', {
             }  
         },
     },
+    mounted () {
+        window.addEventListener('message', this.onMessage, false);
+    },
+    unmounted () {
+        window.removeEventListener('message', this.onMessage, false);
+    },
     methods: {
+        onMessage: function(e) {
+            if (e.data.type && e.data.type == "pretix:widget:title") {
+                this.$el.querySelector("iframe").title = e.data.title;
+            }
+        },
         lightboxClose: function () {
             this.$root.lightbox = null;
         },