[Webkit-unassigned] [Bug 208500] [first paint] visually-non-empty state should match FCP spec

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=208500

--- Comment #4 from Noam Rosenthal <noam at webkit.org> ---
(In reply to Geoffrey Garen from comment #1)
> Note: In cases where the spec and the implementation differ, we need to
> consider suggesting a change to the spec in order to match the
> implementation. It's not necessarily good to change the implementation to
> match the spec, if doing so would make painting worse.
We've had a lot of spec discussions about those differences in the 6 weeks since this comment, and I think we got to a good middle ground and Google had also modified their implementation to conform. For what was left as implementation changes, see rniwa's comment: https://bugs.webkit.org/show_bug.cgi?id=78011#c7. All of this, apart from https://bugs.webkit.org/show_bug.cgi?id=208501 is done.

> 
> Specifically, I'm skeptical about these item:
> 
> - It has a pixel/character threshold, while FCP doesn't
> 
> I'm not sure we want to paint in cases where painting would only render a
> few pixels; to many users, this may seem like a "flash of white", which can
> be bothersome just like a "flash of unstyled content".
The current pixel/character heuristic raise a lot of questions... For example, what does a "few pixels" mean? Currently the pixels of an image are calculated by its intrinsic size, but in some cases it may cover the whole screen if it's extended by CSS. Also a page might show 10 characters of logo and we wouldn't show it because it's less than 200... FCP in that case is less heuristic - if there's anything contentful and potentially visible in the document then the paint is contentful.

But for now, as I understood what was agreed (see above comment link) is that FCP doesn't count pixels/characters but VNE would still block painting

> 
> - It's triggered when the page is ready with no pending requests (this is
> meaningful only for FCP reporting)
> 
> I'm pretty sure we want to paint in cases where there's nothing left to
> load. Though I don't remember the specifics, I remember we faced some cases
> where the entire contents of the webpage were too insignificant to trigger
> any painting according to our VNE heuristics, but we still needed to make
> sure to paint at page load done time, otherwise, we would never paint.

Yes. In this case, FCP might never trigger. I think this is OK, and was discussed on the spec group. FCP may be used to discover bugs, but of course WebKit should display the page in this case.

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