diff --git a/app/channels/presence_channel.rb b/app/channels/presence_channel.rb
index 9c49c13..3ad3bc9 100644
--- a/app/channels/presence_channel.rb
+++ b/app/channels/presence_channel.rb
@@ -22,6 +22,6 @@ class PresenceChannel < RoomChannel
     end
 
     def broadcast_read_room
-      ActionCable.server.broadcast "user_#{current_user.id}_reads", { room_id: membership.room_id }
+      ActionCable.server.broadcast "user_#{current_user.id}_reads", { room_id: membership.room_id, at: Time.current.to_fs(:epoch) }
     end
 end
diff --git a/app/javascript/controllers/read_rooms_controller.js b/app/javascript/controllers/read_rooms_controller.js
index 422808d..ca8f6e2 100644
--- a/app/javascript/controllers/read_rooms_controller.js
+++ b/app/javascript/controllers/read_rooms_controller.js
@@ -16,7 +16,7 @@ export default class extends Controller {
     })
   }
 
-  #read = ({ room_id }) => {
-    this.dispatch("read", { detail: { roomId: room_id } })
+  #read = ({ room_id, at }) => {
+    this.dispatch("read", { detail: { roomId: room_id, at } })
   }
 }
diff --git a/app/javascript/controllers/rooms_list_controller.js b/app/javascript/controllers/rooms_list_controller.js
index afd9981..4868904 100644
--- a/app/javascript/controllers/rooms_list_controller.js
+++ b/app/javascript/controllers/rooms_list_controller.js
@@ -7,6 +7,7 @@ export default class extends Controller {
   static classes = [ "unread" ]
 
   #disconnected = true
+  #readAt = new Map()
 
   async connect() {
     this.channel ??= await cable.subscribeTo({ channel: "UnreadRoomsChannel" }, {
@@ -27,9 +28,13 @@ export default class extends Controller {
     this.read({ detail: { roomId: Current.room.id } })
   }
 
-  read({ detail: { roomId } }) {
+  read({ detail: { roomId, at } }) {
     const room = this.#findRoomTarget(roomId)
 
+    if (at) {
+      this.#readAt.set(Number(roomId), Math.max(Number(at), this.#readAt.get(Number(roomId)) ?? 0))
+    }
+
     if (room) {
       room.classList.remove(this.unreadClass)
       this.dispatch("read", { detail: { targetId: roomId } })
@@ -47,11 +52,11 @@ export default class extends Controller {
     this.#disconnected = true
   }
 
-  #unread({ roomId }) {
+  #unread({ roomId, at }) {
     const unreadRoom = this.#findRoomTarget(roomId)
 
     if (unreadRoom) {
-      if (Current.room.id != roomId) {
+      if (Current.room.id != roomId && !this.#readSince(roomId, at)) {
         unreadRoom.classList.add(this.unreadClass)
       }
 
@@ -59,6 +64,12 @@ export default class extends Controller {
     }
   }
 
+  // Notices fan out one member at a time, so one can arrive after the member has already
+  // read the room in another tab. It still reorders the room, but doesn't mark it unread.
+  #readSince(roomId, at) {
+    return Number(at) <= this.#readAt.get(Number(roomId))
+  }
+
   #findRoomTarget(roomId) {
     return this.roomTargets.find(roomTarget => roomTarget.dataset.roomId == roomId)
   }
diff --git a/app/jobs/message/broadcast_unread_room_job.rb b/app/jobs/message/broadcast_unread_room_job.rb
new file mode 100644
index 0000000..4b0bc2c
--- /dev/null
+++ b/app/jobs/message/broadcast_unread_room_job.rb
@@ -0,0 +1,5 @@
+class Message::BroadcastUnreadRoomJob < ApplicationJob
+  def perform(message)
+    message.broadcast_unread_room
+  end
+end
diff --git a/app/models/message/broadcasts.rb b/app/models/message/broadcasts.rb
index f82e4e3..7118f82 100644
--- a/app/models/message/broadcasts.rb
+++ b/app/models/message/broadcasts.rb
@@ -1,21 +1,29 @@
 module Message::Broadcasts
   def broadcast_create
     broadcast_append_to room, :messages, target: [ room, :messages ]
-    broadcast_unread_room
+    broadcast_unread_room_later
   end
 
   def broadcast_remove
     broadcast_remove_to room, :messages
   end
 
-  private
-    # Fanned out to the room's members rather than published on one global stream, so
-    # that the timing of activity in a room only reaches people who are in it.
-    def broadcast_unread_room
-      payload = ActiveSupport::JSON.encode(roomId: room.id)
+  # Fanned out to the room's members rather than published on one global stream, so
+  # that the timing of activity in a room only reaches people who are in it. Of those,
+  # only the ones who still have it unread or are in it now: by the time this runs,
+  # someone may have opened the room and moved on, and would see it marked unread again.
+  def broadcast_unread_room
+    payload = ActiveSupport::JSON.encode(roomId: room.id, at: created_at.to_fs(:epoch))
 
-      room.memberships.pluck(:user_id).each do |user_id|
-        ActionCable.server.broadcast UnreadRoomsChannel.stream_name_for(user_id), payload, coder: nil
-      end
+    room.memberships.unread.or(room.memberships.connected).pluck(:user_id).each do |user_id|
+      ActionCable.server.broadcast UnreadRoomsChannel.stream_name_for(user_id), payload, coder: nil
+    end
+  end
+
+  private
+    # The fanout is one publish per member, which in a big room takes far longer than the
+    # rest of posting a message, so it runs in a job rather than while the poster waits.
+    def broadcast_unread_room_later
+      Message::BroadcastUnreadRoomJob.perform_later(self)
     end
 end
diff --git a/test/channels/presence_channel_test.rb b/test/channels/presence_channel_test.rb
index 99227a6..881f255 100644
--- a/test/channels/presence_channel_test.rb
+++ b/test/channels/presence_channel_test.rb
@@ -40,6 +40,16 @@ class PresenceChannelTest < ActionCable::Channel::TestCase
     end
   end
 
+  test "subscribing tells the user's other tabs when they read the room" do
+    membership = users(:david).memberships.first
+
+    freeze_time do
+      assert_broadcast_on "user_#{users(:david).id}_reads", { room_id: membership.room_id, at: Time.current.to_fs(:epoch) } do
+        subscribe room_id: membership.room_id
+      end
+    end
+  end
+
   test "unsubscribing marks the membership as disconnected" do
     membership = users(:david).memberships.first
     subscribe room_id: membership.room_id
diff --git a/test/channels/unread_rooms_channel_test.rb b/test/channels/unread_rooms_channel_test.rb
index 98cd236..079234b 100644
--- a/test/channels/unread_rooms_channel_test.rb
+++ b/test/channels/unread_rooms_channel_test.rb
@@ -16,7 +16,9 @@ class UnreadRoomsChannelTest < ActionCable::Channel::TestCase
     assert_not direct.users.include?(users(:jz)), "jz must be an outsider for this test to mean anything"
 
     broadcasts = capture_unread_broadcasts_for(users(:jz)) do
-      direct.messages.create!(body: "Private", creator: users(:kevin), client_message_id: "outsider").broadcast_create
+      perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+        direct.messages.create!(body: "Private", creator: users(:kevin), client_message_id: "outsider").broadcast_create
+      end
     end
 
     assert_empty broadcasts
@@ -24,12 +26,16 @@ class UnreadRoomsChannelTest < ActionCable::Channel::TestCase
 
   test "a member is told about activity in their own room" do
     direct = rooms(:bender_and_kevin)
+    message = nil
 
     broadcasts = capture_unread_broadcasts_for(users(:kevin)) do
-      direct.messages.create!(body: "Private", creator: users(:bender), client_message_id: "member").broadcast_create
+      perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+        message = direct.messages.create!(body: "Private", creator: users(:bender), client_message_id: "member")
+        message.broadcast_create
+      end
     end
 
-    assert_equal [ direct.id ], broadcasts.collect { |broadcast| broadcast["roomId"] }
+    assert_equal [ { "roomId" => direct.id, "at" => message.created_at.to_fs(:epoch) } ], broadcasts
   end
 
   private
diff --git a/test/controllers/messages_controller_test.rb b/test/controllers/messages_controller_test.rb
index e5bfc14..2e20587 100644
--- a/test/controllers/messages_controller_test.rb
+++ b/test/controllers/messages_controller_test.rb
@@ -70,20 +70,50 @@ class MessagesControllerTest < ActionDispatch::IntegrationTest
   end
 
   test "creating a message broadcasts unread room to each member" do
+    memberships(:david_watercooler).present # the poster is in the room
+
     @room.users.each do |member|
       assert_broadcasts UnreadRoomsChannel.stream_name_for(member.id), 1 do
-        post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one #{member.id}", client_message_id: member.id } }
+        perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+          post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one #{member.id}", client_message_id: member.id } }
+        end
       end
     end
   end
 
+  test "creating a message leaves the unread fanout to a job" do
+    member = @room.users.excluding(users(:david)).first
+
+    assert_no_broadcasts UnreadRoomsChannel.stream_name_for(member.id) do
+      post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+    end
+
+    assert_enqueued_with job: Message::BroadcastUnreadRoomJob, args: [ Message.last ]
+  end
+
+  test "the unread fanout skips a member who has opened the room since" do
+    membership = memberships(:jason_watercooler)
+
+    post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+    assert membership.reload.unread?
+
+    membership.present
+    membership.reload.disconnected
+
+    assert_no_broadcasts UnreadRoomsChannel.stream_name_for(membership.user_id) do
+      perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob
+    end
+  end
+
   test "creating a message doesn't broadcast unread room to non-members" do
     outsiders = User.where.not(id: @room.users.map(&:id))
     assert outsiders.any?, "need someone outside the room for this test to mean anything"
 
     outsiders.each do |outsider|
       assert_no_broadcasts UnreadRoomsChannel.stream_name_for(outsider.id) do
-        post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+        perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+          post room_messages_url(@room, format: :turbo_stream), params: { message: { body: "New one", client_message_id: 999 } }
+        end
       end
     end
   end
diff --git a/test/system/unread_rooms_test.rb b/test/system/unread_rooms_test.rb
index 235e912..f2164c4 100644
--- a/test/system/unread_rooms_test.rb
+++ b/test/system/unread_rooms_test.rb
@@ -15,8 +15,11 @@ class UnreadRoomsTest < ApplicationSystemTestCase
     using_session("Kevin") do
       sign_in "kevin@37signals.com"
       join_room designers_room
-      send_message("Hello!!")
-      send_message("Talking to myself?")
+
+      perform_enqueued_jobs only: Message::BroadcastUnreadRoomJob do
+        send_message("Hello!!")
+        send_message("Talking to myself?")
+      end
     end
 
     assert_room_unread designers_room
@@ -24,4 +27,36 @@ class UnreadRoomsTest < ApplicationSystemTestCase
     join_room designers_room
     assert_room_read designers_room
   end
+
+  test "a notice about a message from before the room was read in another tab doesn't mark it unread again" do
+    using_session("Kevin in HQ") do
+      sign_in "kevin@37signals.com"
+      join_room rooms(:hq)
+      broadcast_unread_notice rooms(:designers), to: users(:kevin), at: Time.current
+      assert_room_unread rooms(:designers)
+    end
+
+    sent_before_reading = Time.current
+    using_session("Kevin in Designers") do
+      sign_in "kevin@37signals.com"
+      join_room rooms(:designers)
+    end
+
+    using_session("Kevin in HQ") do
+      assert_room_read rooms(:designers)
+
+      broadcast_unread_notice rooms(:designers), to: users(:kevin), at: sent_before_reading
+      broadcast_unread_notice rooms(:bender_and_kevin), to: users(:kevin), at: Time.current
+      assert_selector "#" + dom_id(rooms(:bender_and_kevin), :list) + ".unread"
+      assert_room_read rooms(:designers)
+
+      broadcast_unread_notice rooms(:designers), to: users(:kevin), at: Time.current
+      assert_room_unread rooms(:designers)
+    end
+  end
+
+  private
+    def broadcast_unread_notice(room, to:, at:)
+      ActionCable.server.broadcast UnreadRoomsChannel.stream_name_for(to.id), { roomId: room.id, at: at.to_fs(:epoch) }
+    end
 end