How to know whether the browser window has the focus or not?
Example:
let has_focus = ref true
let _ =
let open Lwt_js_events in
async (fun () -> focuses (Js.Unsafe.coerce (Dom_html.window))
(fun _ _ -> has_focus := true; Lwt.return ()));
async (fun () -> blurs (Js.Unsafe.coerce (Dom_html.window))
(fun _ _ -> has_focus := false; Lwt.return ()));