htmx plugs ( from https://cosmicrose.dev/blog/htmx-elixir/ )

htmx.ex

# create myapp_web/plugs/htmx.ex
defmodule MyAppWeb.Plugs.Htmx do
  import Plug.Conn
  import Phoenix.Controller, only: [put_root_layout: 2, put_layout: 2, root_layout: 1, layout: 1]

  @behaviour Plug

  @impl true
  def init(opts), do: opts

  @impl true
  def call(conn, _opts) do
    detect_htmx_request(conn)
    htmx_layout(conn)
  end

  defp detect_htmx_request(conn) do
    if get_req_header(conn, "hx-request") == ["true"] do
      assign(conn, :htmx, %{
        request: true,
        boosted: get_req_header(conn, "hx-boosted") != [],
        current_url: List.first(get_req_header(conn, "hx-current-url")),
        history_restore_request: get_req_header(conn, "hx-history-restore-request") == ["true"],
        prompt: List.first(get_req_header(conn, "hx-prompt")),
        target: List.first(get_req_header(conn, "hx-target")),
        trigger_name: List.first(get_req_header(conn, "hx-trigger-name")),
        trigger: List.first(get_req_header(conn, "hx-trigger"))
      })
    else
      conn
    end
  end

  defp htmx_layout(conn) do
    if get_in(conn.assigns, [:htmx, :request]) do
      conn = put_root_layout(conn, html: false)

      if conn.assigns.htmx[:boosted] or conn.assigns.htmx[:history_restore_request] do
        put_layout(conn, layout(conn))
      else
        put_layout(conn, html: false)
      end
    else
      conn
      |> put_root_layout(root_layout(conn))
      |> put_layout(layout(conn))
    end
  end
end

router.ex

defmodule MyAppWeb.Router do
 # ...
 alias MyAppWeb.Plugs
 pipeline :browser do
   plug :accepts, ["html"]
   # ....
   plug Plugs.Htmx
 end