Leave members who are unread already out of a new message's update

Every message rewrote the row of every disconnected member of the room,
including the ones who were unread already and stay unread. Open rooms
take in the whole account, so in steady state that is every member on
every message: 325 WAL pages per post at 10,000 members, against 16 when
only the members who had read the room are written.

Directs keep touching all their members: a direct's sidebar row is
cached by its membership and carries the room's recency, so it has to
be refreshed on every message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017dFSDHrgrLoBELwjV3Qunq

test/controllers/users/sidebars_controller_test.rb

@@ -28,6 +28,23 @@ class Users::SidebarsControllerTest < ActionDispatch::IntegrationTest
     assert_select ".unread", count: users(:david).memberships.reject { |m| m.room.direct? || !m.unread? }.count
   end
 
+  test "a cached direct that is already unread still sorts by its latest message" do
+    room = rooms(:david_and_jason)
+
+    with_memory_cache do
+      travel_to 1.hour.ago do
+        room.messages.create! client_message_id: 998, body: "First", creator: users(:jason)
+      end
+      get user_sidebar_url
+      assert memberships(:david_david_and_jason).reload.unread?
+
+      room.messages.create! client_message_id: 999, body: "Second", creator: users(:jason)
+      get user_sidebar_url
+
+      assert_select "##{dom_id(room, :list)}[data-sorted-list-number=?]", room.reload.updated_at.to_fs(:epoch)
+    end
+  end
+
   test "directs are ordered by room recency, not name" do
     older = rooms(:david_and_jason)
     newer = rooms(:david_and_kevin)
@@ -48,4 +65,23 @@ class Users::SidebarsControllerTest < ActionDispatch::IntegrationTest
     assert positions.all?
     assert_equal positions.sort, positions
   end
+
+  private
+    def with_memory_cache
+      old_cache = Rails.cache
+      old_collection_cache = ActionView::PartialRenderer.collection_cache
+      old_controller_cache = Users::SidebarsController.cache_store
+      old_caching = Users::SidebarsController.perform_caching
+
+      Rails.cache = ActiveSupport::Cache::MemoryStore.new
+      ActionView::PartialRenderer.collection_cache = Rails.cache
+      Users::SidebarsController.cache_store = Rails.cache
+      Users::SidebarsController.perform_caching = true
+      yield
+    ensure
+      Rails.cache = old_cache
+      ActionView::PartialRenderer.collection_cache = old_collection_cache
+      Users::SidebarsController.cache_store = old_controller_cache
+      Users::SidebarsController.perform_caching = old_caching
+    end
 end