FIX: prefills username for new message on first post (#6305)

app/assets/javascripts/select-kit/components/composer-actions.js.es6

@@ -275,6 +275,15 @@ export default DropdownSelectBoxComponent.extend({
       if (postUsername) {
         usernames = postUsername;
       }
+    } else if (this.get("composerModel.topic")) {
+      const stream = this.get("composerModel.topic.postStream");
+
+      if (stream.get("firstPostPresent")) {
+        const post = stream.get("posts.firstObject");
+        if (post && !post.get("yours") && post.get("username")) {
+          usernames = post.get("username");
+        }
+      }
     }
 
     options.action = PRIVATE_MESSAGE;

test/javascripts/acceptance/composer-actions-test.js.es6

@@ -304,3 +304,23 @@ QUnit.test("replying to post as regular user", async assert => {
     );
   });
 });
+
+QUnit.test(
+  "replying to first post - reply_as_private_message",
+  async assert => {
+    const composerActions = selectKit(".composer-actions");
+
+    await visit("/t/internationalization-localization/280");
+    await click("article#post_1 button.reply");
+
+    await composerActions.expand();
+    await composerActions.selectRowByValue("reply_as_private_message");
+
+    assert.equal(find(".users-input .item:eq(0)").text(), "uwe_keim");
+    assert.ok(
+      find(".d-editor-input")
+        .val()
+        .indexOf("Continuing the discussion") >= 0
+    );
+  }
+);