Bug: Firefox asks for confirmation when leaving an exercise that is synchronized

[yurug]

Related issue(s) or PR(s):

• Offer better protections against solution overwriting #372

Bug description

Erik noticed:

Minor issue: when the tab is already synchronized, I think the confirmation dialog This page is asking you to confirm that you want to leave - data you have entered may not be saved. should not appear, while it appears unconditionally(?)

and Yann tried by several means to fix this without success. The documented way that should ensure confirmation deactivation is to delete the returnValue property of the HTML event... but Firefox ignores such deletion.

[erikmd]

So, this issue will be resolved by #372 but just to keep some explanation/details for future reference:

The first implementation version in #372 was working with Chrome:

• 6b63b7d
• https://github.com/ocaml-sf/learn-ocaml/blob/6b63b7d761b828f4f8409d57bfd54dc4d548558f/src/utils/js_utils.ml

  let onbeforeunload ?(win = Dom_html.window) f =
    win##.onbeforeunload := Dom_html.handler (fun ev ->
                                let (status, propagate) = f ev in
                                if status then (
                                   Js.bool propagate
                                ) else
                                  (
                                    Js.(Unsafe.eval_string "undefined");
                                  )
                             )

But Dom.handler appears to be incompatible with "onbeforeunload" and Firefox.

• Namely, the semantics of Dom.handler requires to "return true" to tell Dom.handler that we want to unload normally,
• but with some browsers such as Firefox we should actually "return undefined" (better than "return null" BTW),
• but then, "undefined" is seen as "false", implying Dom.handler will call "preventDefault",
• end_of_the_story.

A sketch of an onbeforeunload-specific handler could be:

  (* Copy of private [Dom.window_event] *)
  let window_event' () : 'a #Dom.event Js.t = Js.Unsafe.pure_js_expr "event" in
  (* Fixed copy of [Dom.handler], working for onbeforeunload
     even for Firefox *)
  let handler_onbeforeunload :
        ((#Js_of_ocaml.Dom_html.event as 'b) Js_of_ocaml__.Js.t ->
         bool Js_of_ocaml__.Js.t) ->
        (* ('a, 'b Js_of_ocaml__.Js.t) Js_of_ocaml.Dom_html.event_listener = *)
        ('a, 'b Js_of_ocaml__.Js.t -> bool Js.t) Js.meth_callback Js.opt =
    fun f ->
  Js.some
    (Js.Unsafe.callback (fun e ->
         (* depending on the internet explorer version, e can be null or undefined. *)
         if not (Js.Opt.test (Js.some e))
         then (
           let e = window_event' () in
           let res = f e in
           if not (Js.to_bool res) then
             (e##.returnValue := res; res)
           else
             Js.Unsafe.pure_js_expr "undefined"
         ) else (
           let res = f e in
           if not (Js.to_bool res) then
             ((Js.Unsafe.coerce e)##preventDefault (* ; res *))
           else
             (Js.Unsafe.delete (Js.Unsafe.coerce e) "returnValue";
              Js.Unsafe.pure_js_expr "undefined")))) in
   *)

Anyway, the current hack is quite simpler:

  let prompt_before_unload () : unit =
    Js.Unsafe.js_expr "window.onbeforeunload = function(e) {e.preventDefault(); return false;}" in
  let resume_before_unload () : unit =
    Js.Unsafe.js_expr "window.onbeforeunload = null" in
  let () =
    Ace.register_sync_observer ace (fun sync ->
        if not sync then prompt_before_unload ()