Remove usage of the unload event

[radnor]

The unload event is deprecated and should not be used. Instead, utilize the visibilitychange or pagehide events.

Additionally, usage of the unload event reflects negatively in Google Lighthouse's SEO scoring.

https://developer.mozilla.org/en-US/docs/Web/API/Window/unload_event

Usage:

aspnetcore/src/Components/Web.JS/src/Boot.Server.Common.ts

Line 100 in f9464dc

window.addEventListener('unload', cleanup, { capture: false, once: true });

[danroth27]

Details on Chrome's plan to deprecate the unload event: https://developer.chrome.com/docs/web-platform/deprecating-unload

[danroth27]

Blazor Server uses the unload to attempt to gracefully cleanup circuits. This is done as an optimization. The timeout and limits on disconnected circuits will still apply.

[danroth27]

Related report on DevExpress Blazor template: https://supportcenter.devexpress.com/ticket/details/t1236027/project-template-unload-event-listeners-are-deprecated-and-will-be-removed-warning-fires

[javiercn]

We probably want to rely on https://developer.mozilla.org/en-US/docs/Web/API/fetchLater_API if possible

[oroztocil]

Considering that we do this only as a resource optimization:

• It is not the end of the world if we don't make the cleanup call (and in practice we currently often don't as unload is notoriously unreliable).
• However, it is really bad if we do the cleanup call prematurely - which seems to make solutions based on visibilitychange unfeasible.

Based on this, we can:

• Use fetchLater in supported browsers. We can update the circuit ID in the queued payload on circuit resume by aborting the request and queueing a new one (is there another way?).
• In browser without support for fetchLater, use the current solution with the pagehide event instead of unload. pagehide is supposed to be similarly unreliable as unload but it is not getting deprecated and does not break the back/forward cache. This would still often not work on mobile but should be useful on e.g. desktop Safari and Firefox.

@javiercn Thoughts?