Server Sent Events - https does not work: net::ERR_HTTP2_PROTOCOL_ERROR

Hello,

I am trying to get server sent events to work with Phoenix. And I can get it to work successfully over http.
Following are the steps i followed -
Add mime type to config.exs

config :mime, :types, %{
  "text/event-stream" => ["sse"]
}

In my controller I have

defmodule MyAppWeb.NotificationController do
  use MyAppWeb, :controller
  # alias Phoenix.PubSub

  def index(conn, _params) do
    conn =
      conn
      |> put_resp_header("Cache-Control", "no-cache")
      |> put_resp_header("connection", "keep-alive")
      |> put_resp_header("Content-Type", "text/event-stream; charset=utf-8")
      |> put_resp_header("Access-Control-Allow-Origin", "*")
      |> send_chunked(200)

    IO.puts("===============")

    PubSub.subscribe(
      self(),
      :notification
    )
    |> IO.inspect()

    IO.puts("===============")

    IO.inspect(self())
    sse_loop(conn, self())
  end

  defp sse_loop(conn, pid) do
    # receive is what stops the router from processing
    # and waits for an event to come in
    receive do
      # Send updates when :cagg is finished.
      {:notification, :done} ->
        # Fetch notification
        IO.inspect("*************************")
        # Send update.
        chunk(conn, "data: message\n\n")
        # Wait for next publish.
        sse_loop(conn, pid)

      # Stop SSE if this conn is actually down.
      # Ignore other processes finishing.
      # Notice the "^" in front of pid.
      {:DOWN, _reference, :process, ^pid, _type} ->
        nil

      # Don't stop SSE because of unrelated events.
      other -> 
        sse_loop(conn, pid)
    end
  end
end

In the router I have

get "/notification", NotificationController, :index

Also need to set idle_timeout for cowboy to infinity otherwise it times out after 60secs
So in the dev.exs file I have

http: [port: 8066, compress: true, protocol_options: [idle_timeout: :infinity]],
  https: [
    port: 8077,
    compress: true,
    cipher_suite: :strong,
    keyfile: "priv/keys/localhost.key",
    certfile: "priv/keys/localhost.cert",
    protocol_options: [idle_timeout: :infinity]
  ],

In the template I have simple javascript for SSE

<script>
    let eventSource = new EventSource("/notification");

    eventSource.onmessage = function(event) {
      console.log("New message", event.data);
    };
  </script>

I got great help from this blog - Server Sent Events with Elixir

Everything works over http. However for https it doesn’t work.
I get this error
GET https://localhost:8077/notification net::ERR_HTTP2_PROTOCOL_ERROR

Request your kind help.