Take a look at wxdisplay, it should be exactly what you are looking for.

From the docs (wxDisplay — OTP 29.0.2 (wx 2.6)) I understand I need to:

_wx = :wx.new()
display = :wxDisplay.new()
{x, y, w, h} = :wxDisplay.getClientArea(display)

Being the first time I am using this library, do I really need the first line _wx = :wx.new()?
Am I missing something?

No idea, read some documentation on how this is done in wx, as this is just a erlang wrapper for wx library.

My guess is that it starts a process responsible for getting messages from requests, as in, a central point where everything has to go through. This is based on the docs:

However, I could not find any confirmation if my interpretation is correct, thus I asked.

Yes, you’re right. you can typically also get the same effect by using :wx.set_env(env) using an existing wx_env for this there is a helper in Desktop.Env: Desktop.Env — Desktop v1.5.3

:wx.set_env(Desktop.Env.wx_env())
display = :wxDisplay.new()
{x, y, w, h} = :wxDisplay.getClientArea(display)

In my specific use case, I cannot use :wx.set_env(Desktop.Env.wx_env()) as the Desktop application has not yet started:

See my application.ex:

 def start(_type, _args) do
    children = [
      Telemetry,
      {Phoenix.PubSub, name: PubSub},
      Endpoint,
      Manager,
      %{
        id: Desktop.Window,
        start: {Desktop.Window, :start_link, [[
          app: :web_interface,
          id: WebInterface,
          title: "Market Manager",
          size: calculate_window_size(0.4, 0.8),
          menubar: MenuBar,
          icon: "static/images/resized_logo_5_32x32.png",
          url: fn -> "#{WebInterface.Endpoint.url()}/#{@landing_page}" end
        ]]},
        restart: :transient,
        shutdown: 5_000
      }
    ]

    opts = [strategy: :one_for_one, name: WebInterface.Supervisor]
    Supervisor.start_link(children, opts)
  end

  @doc """
  Calculates the width and height for the window. Fetches the display
  information using :wx. Because we create a :wx server every time we call this
  function this is a heavy operation and should be used cautiously.

  Both percentages given must be > 0 and <= 1.
  """
  @spec calculate_window_size(float, float) :: {pos_integer, pos_integer}
  def calculate_window_size(width_percentage, height_percentage)
      when is_float(width_percentage) and width_percentage > 0 and width_percentage <= 1 and
             is_float(height_percentage) and height_percentage > 0 and height_percentage <= 1 do
    # Since Desktop has not yet started its wx server, we need to create one
    # to get the screen size first
    _wx_object = :wx.new()

    display = :wxDisplay.new()
    {_x, _y, total_width, total_height} = :wxDisplay.getClientArea(display)

    width_pixels = ceil(width_percentage * total_width)
    height_pixels = ceil(height_percentage * total_height)

    # we destroy the previously created wx server to save resources and avoid
    # a conflict with Desktop library
    :wx.destroy()

    {width_pixels, height_pixels}
  end

If I replace _wx_object = :wx.new() with :wx.set_env(Desktop.Env.wx_env()) then the application crashes on startup:

07:24:00.603 [notice] Application wx exited: :stopped
07:24:00.606 [notice] Application runtime_tools exited: :stopped
** (Mix) Could not start application web_interface: WebInterface.Application.start(:normal, []) returned an error: shutdown: failed to start child: Desktop.Window
    ** (EXIT) an exception was raised:
        ** (MatchError) no match of right hand side value: {:error, {:noproc, {:gen_server, :call, [#PID<0.717.0>, :register_me, :infinity]}}}
            (desktop 1.5.1) lib/desktop/window.ex:109: Desktop.Window.start_link/1
            (stdlib 4.1.1) supervisor.erl:414: :supervisor.do_start_child_i/3
            (stdlib 4.1.1) supervisor.erl:400: :supervisor.do_start_child/2
            (stdlib 4.1.1) supervisor.erl:384: anonymous fn/3 in :supervisor.start_children/2
            (stdlib 4.1.1) supervisor.erl:1250: :supervisor.children_map/4
            (stdlib 4.1.1) supervisor.erl:350: :supervisor.init_children/2
            (stdlib 4.1.1) gen_server.erl:851: :gen_server.init_it/2
            (stdlib 4.1.1) gen_server.erl:814: :gen_server.init_it/6

Therefore, the solution I found (and works for me) is to create a temporary wx_object and then kill it, to avoid conflicts with Desktop, just like in the code snippet I shared above.

This also reminds me of one question:

• @dominicletz what do you think aboud adding a function that takes in percentages for the windows size instead of absolute pixel values, like the function I created?