Task: Integrate feedback from https://github.com/ocsigen/js_of_ocaml/issues/1436

[erikmd]

Related user(s):

@erikmd, @hhugo

Related issue(s) or PR(s):

• [BUG] semantics of (onbeforeunload, Dom_html.handler) incompatible with major browsers ocsigen/js_of_ocaml#1436

Related project scope(s):

web-app UI

Question?

• TODO: Apply @hhugo's suggestion to avoid using Js.Unsafe.js_expr.
• Next: Understand whether we can avoid Obj.magic and/or directly use (a modified version of) Dom_html.handler

What has already been tested:

No response

learn-ocaml --version

No response

git describe --long --always --abbrev=40 --tags

No response

What OS do you use?

GNU/Linux

What browser(s) do you use with learn-ocaml?

No response

Additional context:

No response