Collapse/hide resolved discussions

app/models/discussion.rb

@@ -87,8 +87,20 @@ class Discussion
     self.noteable == target && !diff_discussion?
   end
 
+  def collapsed?
+    return false unless diff_discussion?
+
+    if resolvable?
+      # New diff discussions only disappear once they are marked resolved
+      resolved?
+    else
+      # Old diff discussions disappear once they become outdated
+      !active?
+    end
+  end
+
   def expanded?
-    !diff_discussion? || active?
+    !collapsed?
   end
 
   def reply_attributes

app/models/note.rb

@@ -86,8 +86,10 @@ class Note < ActiveRecord::Base
     end
 
     def grouped_diff_discussions
-      notes = diff_notes.fresh.select(&:active?)
-      Discussion.for_diff_notes(notes).map { |d| [d.line_code, d] }.to_h
+      active_notes = diff_notes.fresh.select(&:active?)
+      Discussion.for_diff_notes(active_notes).
+        reject(&:resolved?).
+        map { |d| [d.line_code, d] }.to_h
     end
 
     # Searches for notes matching the given query.