I have been playing around with ex_termbox (which provides bindings to termbox) as a way to “paint” the screen when running CLI apps. I’ve got most things working – the package has some excellent examples. However I’m not sure how to “clear” the screen when I launch something from iex. My loop is all in the context of a GenServer – the important parts look something like this:

use GenServer

alias ExTermbox.Bindings, as: Termbox
alias ExTermbox.{Cell, EventManager, Event, Position}

def start_link(%{name: name} = state) do
  GenServer.start_link(__MODULE__, %{response: ""}, name: name)
end

@impl true
def init(%{name: name}) do

  :ok = Termbox.init()

  {:ok, em_pid} = EventManager.start_link(name: :"#{name}_event_mgr")
  :ok = EventManager.subscribe(em_pid, name)

  {:ok, state, {:continue, :draw_screen}}
end

  @impl true
  def handle_continue(:draw_screen, %{response: response} = state) do

    Termbox.clear()

    response = String.to_charlist(response)

    for {ch, x} <- Enum.with_index(response) do
      :ok = Termbox.put_cell(%Cell{position: %Position{x: x, y: 0}, ch: ch})
    end

    Termbox.present()

    {:noreply, state}
  end

However, when I start an instance of this GenServer from iex, the screen is not fully cleared. Things work, but I end up seeing parts of the iex session “bleeding through”. Something like this:

❯ █  ok, #PID<0.200.0>}
iex(2)>

where I can type input at the ❯ prompt and things work, but it just looks wrong. Things are a bit better if I manually clear the screen, e.g. via my own function like this:

  defp clear_screen do
    {:ok, width} = Termbox.width()
    {:ok, height} = Termbox.height()

    for x <- 0..width, y <- 0..height do
      :ok = Termbox.put_cell(%Cell{position: %Position{x: x, y: y}, ch: 8206})
    end
  end

If I use a unicode whitespace character (e.g. ch 8206), then most of the screen gets covered like I want and the iex> prompt and pid are visible only at the very bottom of the screen.

My guess is that iex and termbox are wrestling for control over the current view port. Is there a way for termbox to fully take control over the screen, or am I bound to accommodate iex so long as I am launching my app from within iex?

Thanks for any pointers, thoughts, enlightenment.