Standardizing rendering pipeline / refresh cycle

"L. David Baron" <[email protected]> Sat, 29 Aug 2015 15:58:24 +0200
Newsgroups	gmane.comp.mozilla.devel.layout
Message-ID	<[email protected]>
At today's Houdini task force meeting:
https://wiki.css-houdini.org/planning/paris-2015 some of the blink folks
(mainly Ojan Vafai) presented the outline of a proposal for
standardizing some aspects of the rendering pipeline / refresh cycle:
https://docs.google.com/document/d/1Mw6qNw8UAEfW96CXaXRVYPPZjqQS3YdK7v57wFttAhs/edit
which would probably replace at least parts of this bit of HTML:
https://html.spec.whatwg.org/multipage/webappapis.html#processing-model-9

Some of the goals of this would be to improve interop in terms of:

 * which states of a document can be painted (e.g., if there's a
   setInterval that makes the background red and a requestAnimationFrame
   that makes the background green, is it ever possible for the
   background to show as red?  Blink proposes the answer should be no)

 * ordering of various events

 * what the current state is when various events fire (e.g., scroll
   events, animation events)

The goal would be to agree on this stuff and evolve to be interoperable
over a reasonably long time frame.


My initial sense is that it seems reasonable to standardize, and we
should work out the details.


I think one of the harder changes for the concrete pieces that they've
proposed so far is that we'd need to make is not calling into our
painting code from anywhere other than nsRefreshDriver::Tick, i.e.,
eliminating the callers in nsViewManager::UpdateWidgetGeometry and
nsViewManager::WillPaintWindow.  At least, that's based on their initial
reports that a bunch of browsers do sometimes display red in the
scenario above.

The current proposal is to work on this in the Houdini task force, since
the right people seem to be there, even though it isn't quite a perfect
fit for the mission of the task force.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)

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