Add indicator of unread content to window title when web UI is out of focus (#11560)

Fix #1288

app/javascript/mastodon/reducers/missed_updates.js

@@ -0,0 +1,23 @@
+import { Map as ImmutableMap } from 'immutable';
+import { NOTIFICATIONS_UPDATE } from 'mastodon/actions/notifications';
+import { TIMELINE_UPDATE } from 'mastodon/actions/timelines';
+import { APP_FOCUS, APP_UNFOCUS } from 'mastodon/actions/app';
+
+const initialState = ImmutableMap({
+  focused: true,
+  unread: 0,
+});
+
+export default function missed_updates(state = initialState, action) {
+  switch(action.type) {
+  case APP_FOCUS:
+    return state.set('focused', true).set('unread', 0);
+  case APP_UNFOCUS:
+    return state.set('focused', false);
+  case NOTIFICATIONS_UPDATE:
+  case TIMELINE_UPDATE:
+    return state.get('focused') ? state : state.update('unread', x => x + 1);
+  default:
+    return state;
+  }
+};