Change mergeInto return signature; only merge posts if same user

framework/core/src/Core/Models/DiscussionRenamedPost.php

@@ -13,16 +13,21 @@ class DiscussionRenamedPost extends ActivityPost
      * Merge the post into another post of the same type.
      *
      * @param \Flarum\Core\Models\DiscussionRenamedPost $previous
-     * @return boolean true if the post was merged, false if it was deleted.
+     * @return \Flarum\Core\Models\Model|null The final model, or null if the
+     *     previous post was deleted.
      */
     protected function mergeInto(Model $previous)
     {
+        if ($this->user_id === $previous->user_id) {
             if ($previous->content[0] == $this->content[1]) {
-            return false;
+                return;
             }
 
             $previous->content = static::buildContent($previous->content[0], $this->content[1]);
-        return true;
+            return $previous;
+        }
+
+        return $this;
     }
 
     /**

framework/core/src/Core/Models/MergeableTrait.php

@@ -5,14 +5,14 @@ trait MergeableTrait
     public function saveAfter(Model $previous)
     {
         if ($previous instanceof static) {
-            if ($this->mergeInto($previous)) {
-                $previous->save();
+            if ($merged = $this->mergeInto($previous)) {
+                $merged->save();
+                return $merged;
             } else {
                 $previous->delete();
-            }
-
                 return $previous;
             }
+        }
 
         $this->save();