Re: [PageVisibility] ALT+TAB behaviour

In line with what Boris says, it is very hard for us to know when a window
is covered by another window. Not every OS is consistent on telling windows
about occlusion --- it is computationally expensive to figure out
occlusion. We can know in some extreme cases: minimizing, for instance.
But, there are a lot of corner cases: when a window goes to a hidden
workspace, for instance, some OS/window managers tell us, but some don't.

The page visibility spec is, I recall, crafted carefully, to allow false
positives for "visible" but not false negatives.

That is, "page is visible" means "the page may be visible to the user, but
user agent doesn't have to be 100% sure."

But, "page is not visible" means "the page is definitely not visible to the
user." This is the very important one for us to get right.