Add client "close" event

[AshleyScirra]

If a SW wants to map a clientId to some state, there's no good way to know when to remove the map key. Adding an event that is fired when a client is closed would make this simpler.

Note you can't use a WeakMap, since clients.get() returns a different Client object every time even for the same clientId, so the same client gets added as new keys every time (which IMO is a nasty gotcha).

[jakearchibald]

We should consider this feature.

In the mean time, performing GC like this when you see navigation fetches works pretty well

[wanderview]

It would need to be slightly different from "close". Currently the spec controls windows, not documents. So a client can be closed, but also just navigated to a non-controlled document. In that case the client still exists, but is no longer a controlling client.

Perhaps we could add events to Client for transition controlled vs non-controlled state.

[NekR]

Also Ctrl/Cmd + Shift + T (re-open tab) restores previous state of the page & back/forward history navigation can perform just session state restore. So this cannot be just close event, because then you will need an open[-again] event.

[jakearchibald]

#886 seems to have more detail