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[cssom-view] Expose the 'renderport' to content

From: Matt Woodrow <mwoodrow@mozilla.com>
Date: Wed, 6 Apr 2016 23:11:22 +1200
To: "www-style@w3.org" <www-style@w3.org>
Cc: Kartikaya Gupta <kgupta@mozilla.com>
Message-ID: <5704EEDA.6030003@mozilla.com>

I'd like to propose exposing the 'renderport' to web content, defined as 
the area of a scrollable container that is actually rendered by the UA, 
which may be larger than the viewport (visible) area.

Most browsers these days are rendering areas larger than what is 
currently visible (especially, but not limited to the root scrollframe) 
to allow asynchronous scrolling.

We also have a lot examples (especially on mobile) of web content that 
dynamically adds and removes content based on the scroll position 
(referred to as 'data scrollers' in the IntersectionObserver spec).

Both of these systems are independently maintaining a definition of what 
is 'soon to be visible', and when they aren't in sync we get sub-optimal 
performance. If the data scroller chooses an area larger than the UA, 
then they are bloating the DOM unnecessarily, which is what they are 
trying to avoid. If the data scroller chooses an area smaller than the 
UA, then we get unnecessary invalidations and repeated drawing of the 
same pixels.

Our suggested solution to this is to add a callback to scrollable 
elements that fires before painting (similar to requestAnimationFrame) 
and exposes the (approximate) region of the element that the UA is going 
to treat as visible for the purpose of painting.

DOM changes from within the callback that result in the final renderport 
area changing probably shouldn't trigger another callback until after 
the paint to avoid infinite loops.

It might also be nice to extend the IntersectionObserver spec to allow 
specifying the renderport (plus a margin) as the intersection region to 
observe, so that content can trigger asynchronous loading of data before 
it enters the renderport, but that can wait until we make a decision on 

- Matt
Received on Wednesday, 6 April 2016 11:11:56 UTC

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