UX: Invited users should watch PM topic once topic has been visited.

https://meta.discourse.org/t/notifications-not-received-for-private-messages-im-invited-to/71577/11

spec/models/topic_user_spec.rb

@@ -234,13 +234,38 @@ describe TopicUser do
     end
 
     context 'private messages' do
+      let(:target_user) { Fabricate(:user) }
+
+      let(:post) do
+        create_post(
+          archetype: Archetype.private_message,
+          target_usernames: target_user.username
+        );
+      end
+
+      let(:topic) { post.topic }
+
       it 'should ensure recepients and senders are watching' do
+        expect(TopicUser.get(topic, post.user).notification_level)
+          .to eq(TopicUser.notification_levels[:watching])
 
-        target_user = Fabricate(:user)
-        post = create_post(archetype: Archetype.private_message, target_usernames: target_user.username);
+        expect(TopicUser.get(topic, target_user).notification_level)
+          .to eq(TopicUser.notification_levels[:watching])
+      end
 
-        expect(TopicUser.get(post.topic, post.user).notification_level).to eq(TopicUser.notification_levels[:watching])
-        expect(TopicUser.get(post.topic, target_user).notification_level).to eq(TopicUser.notification_levels[:watching])
+      it 'should ensure invited user is watching once visited' do
+        another_user = Fabricate(:user)
+        topic.invite(target_user, another_user.username)
+        TopicUser.track_visit!(topic.id, another_user.id)
+
+        expect(TopicUser.get(topic, another_user).notification_level)
+          .to eq(TopicUser.notification_levels[:watching])
+
+        another_user = Fabricate(:user)
+        TopicUser.track_visit!(topic.id, another_user.id)
+
+        expect(TopicUser.get(topic, another_user).notification_level)
+          .to eq(TopicUser.notification_levels[:regular])
       end
     end