Safari doesn't fire visibilitychange event on the page close

[alekseykulikov]

web-vitals-reporter uses onHidden utility from web-vitals to detect the end of the session, which relies on a visibilitychange event.

Safari doesn't fire visibilitychange on the page close or refresh (similar issue).

It's possible to use pagehide event, but it fires before visibilitychange in Chromium/Firefox, and there's a potential of losing LCP/CLS values, which depends on onHidden.

@philipwalton, what do you think about this? Is it possible to detect proper support of visibilitychange and fallback to pagehide if not?

[alekseykulikov]

The fix I ended up with: if a browser supports CLS/LCP, try to use onHidden; otherwise, use pagehide.