initial_present needs to be run before subscribing to @presence_topic to pass the test

[rizdarmwn]

right now the suggested code to add on the Presence part is

defmodule CounterWeb.Counter do
  use CounterWeb, :live_view
  alias Counter.Count
  alias Phoenix.PubSub
+ alias Counter.Presence

  @topic Count.topic
+ @presence_topic "presence"

  def mount(_params, _session, socket) do
    PubSub.subscribe(Counter.PubSub, @topic)

+   Presence.track(self(), @presence_topic, socket.id, %{})
+   CounterWeb.Endpoint.subscribe(@presence_topic)
+
+   initial_present =
+     Presence.list(@presence_topic)
+     |> map_size

+   {:ok, assign(socket, val: Count.current(), present: initial_present) }
-   {:ok, assign(socket, val: Count.current()) }
  end

  def handle_event("inc", _, socket) do
    {:noreply, assign(socket, :val, Count.incr())}
  end

  def handle_event("dec", _, socket) do
    {:noreply, assign(socket, :val, Count.decr())}
  end

  def handle_info({:count, count}, socket) do
    {:noreply, assign(socket, val: count)}
  end

+ def handle_info(
+       %{event: "presence_diff", payload: %{joins: joins, leaves: leaves}},
+       %{assigns: %{present: present}} = socket
+    ) do
+   new_present = present + map_size(joins) - map_size(leaves)
+
+   {:noreply, assign(socket, :present, new_present)}
+ end

  def render(assigns) do
    ~H"""
    <.live_component module={CounterComponent} id="counter" val={@val} />
+   <.live_component module={PresenceComponent} id="presence" present={@present} />
    """
  end
end

where it should be

defmodule CounterWeb.Counter do
  use CounterWeb, :live_view
  alias Counter.Count
  alias Phoenix.PubSub
+ alias Counter.Presence

  @topic Count.topic
+ @presence_topic "presence"

  def mount(_params, _session, socket) do
    PubSub.subscribe(Counter.PubSub, @topic)

+   Presence.track(self(), @presence_topic, socket.id, %{})
+   initial_present =
+     Presence.list(@presence_topic)
+     |> map_size
+   CounterWeb.Endpoint.subscribe(@presence_topic)

+   {:ok, assign(socket, val: Count.current(), present: initial_present) }
-   {:ok, assign(socket, val: Count.current()) }
  end

  def handle_event("inc", _, socket) do
    {:noreply, assign(socket, :val, Count.incr())}
  end

  def handle_event("dec", _, socket) do
    {:noreply, assign(socket, :val, Count.decr())}
  end

  def handle_info({:count, count}, socket) do
    {:noreply, assign(socket, val: count)}
  end

+ def handle_info(
+       %{event: "presence_diff", payload: %{joins: joins, leaves: leaves}},
+       %{assigns: %{present: present}} = socket
+    ) do
+   new_present = present + map_size(joins) - map_size(leaves)
+
+   {:noreply, assign(socket, :present, new_present)}
+ end

  def render(assigns) do
    ~H"""
    <.live_component module={CounterComponent} id="counter" val={@val} />
+   <.live_component module={PresenceComponent} id="presence" present={@present} />
    """
  end
end

where initial_present is before subscribing to @presence_topic. this also referred to the actual code on the repo.
if it's after, the test won't pass for handle_info/2 presence updates.