[feature request] onClose event

[acarnwath]

There is currently an onCancel event and an onInstall event -- the onInstall fires when the user accepts the message in the "banner" but on non-native browsers, a second popup appears with instructions, so there is currently no built-in way to detect when the popups are gone (additionally they timeout, etc.)

I would like this event as I am adding in aria labels for accessibility's sake and I don't know when to flip it to aria-hidden. I'm also styling it as a takeover popup, and adding a class to the body to prevent scrolling, but I need to know when to remove it.