FIX: puts focus-event at application level (#7568)

Also attempts to use simpler and newer APIs

app/assets/javascripts/discourse-common/mixins/focus-event.js.es6

@@ -0,0 +1,36 @@
+function gotFocus() {
+  if (!Discourse.get("hasFocus")) {
+    Discourse.setProperties({ hasFocus: true, notify: false });
+  }
+}
+
+function lostFocus() {
+  if (Discourse.get("hasFocus")) {
+    Discourse.set("hasFocus", false);
+  }
+}
+
+let onchange;
+
+export default Ember.Mixin.create({
+  ready() {
+    this._super(...arguments);
+
+    onchange = () => {
+      document.visibilityState === "hidden" ? lostFocus() : gotFocus();
+    };
+
+    // Default to true
+    Discourse.set("hasFocus", true);
+
+    document.addEventListener("visibilitychange", onchange);
+  },
+
+  reset() {
+    this._super(...arguments);
+
+    document.removeEventListener("visibilitychange", onchange);
+
+    onchange = undefined;
+  }
+});