Clarify prerendering documents cannot have focus.

[Clqsin45]

Some APIs's specs[1][2] require documents to have focus[3] when using these APIs. So for prerendering, maybe we should clarify that prerendering documents cannot be focused and executing document.hasFocus() on these pages always returns false[4].

Maybe we should also clarify that prerendering browsing context cannot be active?

cc: @nhiroki

[1] https://w3c.github.io/clipboard-apis/#privacy-async
[2] https://www.w3.org/TR/pointerlock/#extensions-to-the-element-interface
[3] https://html.spec.whatwg.org/multipage/interaction.html#focus
[4] https://chromium-review.googlesource.com/c/chromium/src/+/2743865/5..9/third_party/blink/web_tests/wpt_internal/prerender/resources/document-has-focus.html#b17

[jeremyroman]

@a4sriniv might have thoughts about focus in prerendering/portal contexts

[domenic]

We only very recently added the concept of system focus to the HTML spec. Although, we didn't hook it up to document.hasFocus(), hmm. We did, I just forgot.

My take, based on previous discussions, is that prerendering browsing contexts can have their contents focused (e.g., they can have a nontrivial document.activeElement; someElement.focus() works; autofocus="" works). But, they cannot have system focus.

[a4sriniv]

I agree with @domenic, prerendering browsing contexts should behave the same as portals do currently (they don't have system focus and only get it on activation). See WICG/portals#257 for the portals discussion on focus.

[Clqsin45]

Thank you for discussing!
I've added wpt-internal tests.

[mfalken]

Thanks @Clqsin45 for raising this. We could probably extend the section on Activation-gated APIs to also cover System Focus-gated APIs:
https://jeremyroman.github.io/alternate-loading-modes/#activation-gated

[mfalken]

It appears some APIs like Picture-In-Picture may also be gated on visibility state as well as user activation (e.g., https://w3c.github.io/picture-in-picture/#auto-pip can be used without user activation if visibility state changes??), so we may also add visibility state to this section. Filed related #59

[mfalken]

This is fixed in #62 and the Clipboard case specifically in #69. Thanks @Clqsin45 for raising this!