Paint metrics are reported when page starts off in a hidden state

[kylewm]

Hi! I noticed a bug in my own application's metric tracking this morning, went to check how Perfume solved it, and I think I found that Perfume has the same bug:

If I open a link in a new tab, wait a few seconds, and then switch to the tab, FCP and LCP are still reported, but the duration is the time I waited before switching to the tab.

I believe this is because visibility.isHidden is only set to true by a visibilitychange event, and in this case there is no event fired until I switch to the tab and page becomes visible.

Here's a simple example demonstrating the issue: https://kylewm.github.io/perfume-demo/

[Zizzamia]

Oh interesting, great catch! I will do some more investigation on this and see how to fix it.
If you have any idea on how to, feel free to open a PR.