[whatwg] prompts, alerts and showModalDialog during beforeunload/unload events

[Mike Hearn]

Browsers could solve the editor use case by treating "close tab" as
"hide tab" for a minute or two before actually shutting down the page.

Then the problem becomes, how do you make it obvious to users that
they can get their work back by pressing a magic button somewhere?

The modal quit loop is frequently used to try and make people download
malware. It'd be nice to eliminate it in a backwards compatible
manner.