diff --git a/app/models/room.rb b/app/models/room.rb
index ed7280d..aaca7d5 100644
--- a/app/models/room.rb
+++ b/app/models/room.rb
@@ -104,8 +104,13 @@ class Room < ApplicationRecord
       end
     end
 
+    # Rewriting every member on every message is most of what posting to a large room writes,
+    # so members who are unread already stay as they are. Directs keep touching them all: a
+    # direct's sidebar row is cached by membership and shows the room's recency.
     def unread_memberships(message)
-      memberships.visible.disconnected.where.not(user: message.creator).update_all(unread_at: message.created_at, updated_at: Time.current)
+      recipients = memberships.visible.disconnected.where.not(user: message.creator)
+      recipients = recipients.where(unread_at: nil) unless direct?
+      recipients.update_all(unread_at: message.created_at, updated_at: Time.current)
     end
 
     def push_later(message)
diff --git a/test/controllers/users/sidebars_controller_test.rb b/test/controllers/users/sidebars_controller_test.rb
index c563541..dd8ad03 100644
--- a/test/controllers/users/sidebars_controller_test.rb
+++ b/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
diff --git a/test/models/room_test.rb b/test/models/room_test.rb
index 09771f3..306a38f 100644
--- a/test/models/room_test.rb
+++ b/test/models/room_test.rb
@@ -25,6 +25,20 @@ class RoomTest < ActiveSupport::TestCase
     assert room.users.include?(users(:david))
   end
 
+  test "a new message marks unread the members who had read the room and leaves the unread ones as they were" do
+    room = rooms(:watercooler)
+    reader = memberships(:jason_watercooler)
+    behind = memberships(:bender_watercooler)
+    behind.update_columns unread_at: 1.hour.ago, updated_at: 1.hour.ago
+    behind_before = behind.reload.attributes.slice("unread_at", "updated_at")
+
+    message = room.messages.create! creator: users(:david), body: "Hello", client_message_id: "unread-once"
+
+    assert_equal message.created_at, reader.reload.unread_at
+    assert_equal behind_before, behind.reload.attributes.slice("unread_at", "updated_at")
+    assert_not memberships(:david_watercooler).reload.unread?
+  end
+
   test "type" do
     assert Rooms::Open.new.open?
     assert_not Rooms::Open.new.direct?