ForceMentionsInContent: don't apply it to top-level posts

lib/pleroma/web/activity_pub/mrf/force_mentions_in_content.ex

@@ -72,8 +72,13 @@ defmodule Pleroma.Web.ActivityPub.MRF.ForceMentionsInContent do
   end
 
   @impl true
-  def filter(%{"type" => "Create", "object" => %{"type" => "Note", "to" => to}} = object)
-      when is_list(to) do
+  def filter(
+        %{
+          "type" => "Create",
+          "object" => %{"type" => "Note", "to" => to, "inReplyTo" => in_reply_to}
+        } = object
+      )
+      when is_list(to) and is_binary(in_reply_to) do
     # image-only posts from pleroma apparently reach this MRF without the content field
     content = object["object"]["content"] || ""
 

test/pleroma/web/activity_pub/mrf/force_mentions_in_content_test.exs

@@ -109,4 +109,29 @@ defmodule Pleroma.Web.ActivityPub.MRF.ForceMentionsInContentTest do
     {:ok, %{"object" => %{"content" => filtered}}} = ForceMentionsInContent.filter(activity)
     assert filtered == "I'ma tired..."
   end
+
+  test "don't mention in top-level posts" do
+    mario = insert(:user, nickname: "mario")
+    luigi = insert(:user, nickname: "luigi")
+
+    {:ok, post} = CommonAPI.post(mario, %{status: "Letsa go"})
+
+    activity = %{
+      "type" => "Create",
+      "actor" => mario.ap_id,
+      "object" => %{
+        "type" => "Note",
+        "actor" => mario.ap_id,
+        "content" => "Mama mia!",
+        "to" => [
+          luigi.ap_id,
+          Constants.as_public()
+        ],
+        "quoteUrl" => Object.normalize(post).data["id"]
+      }
+    }
+
+    {:ok, %{"object" => %{"content" => filtered}}} = ForceMentionsInContent.filter(activity)
+    assert filtered == "Mama mia!"
+  end
 end