FIX: tag intersection not populating (#6296)

* Fix for tag intersection not populating

* Fix prettier

* Add acceptance test for tags intersection

app/assets/javascripts/discourse/routes/tags-show.js.es6

@@ -184,9 +184,11 @@ export default Discourse.Route.extend({
               var c = self.controllerFor("composer").get("model");
               c.set(
                 "tags",
-                _.flatten(
-                  [controller.get("model.id")],
+                _.compact(
+                  _.flatten([
+                    controller.get("model.id"),
                     controller.get("additionalTags")
+                  ])
                 )
               );
             }

test/javascripts/acceptance/tags-intersection-test.js.es6

@@ -0,0 +1,43 @@
+import { acceptance } from "helpers/qunit-helpers";
+
+acceptance("Tags intersection", {
+  loggedIn: true,
+  site: { can_tag_topics: true },
+  settings: { tagging_enabled: true },
+  pretend(server, helper) {
+    server.get("/tags/first/notifications", () => {
+      return helper.response({
+        tag_notification: { id: "first", notification_level: 1 }
+      });
+    });
+    server.get("/tags/intersection/first/second.json", () => {
+      return helper.response({
+        users: [],
+        primary_groups: [],
+        topic_list: {
+          can_create_topic: true,
+          draft_key: "new_topic",
+          topics: [{ id: 16, posters: [] }],
+          tags: [
+            { id: 1, name: "first", topic_count: 1 },
+            { id: 2, name: "second", topic_count: 1 }
+          ]
+        }
+      });
+    });
+  }
+});
+
+QUnit.test("Populate tags when creating new topic", async assert => {
+  await visit("/tags/intersection/first/second");
+  await click("#create-topic");
+
+  assert.ok(exists(".mini-tag-chooser"), "The tag selector appears");
+  assert.equal(
+    $(".mini-tag-chooser")
+      .text()
+      .trim(),
+    "first, second",
+    "populates the tags when clicking 'New topic'"
+  );
+});