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[css-overflow][css-scroll-snap][css-position] position: fixed and paging

From: fantasai <fantasai.lists@inkedblade.net>
Date: Tue, 9 Feb 2016 22:14:58 -0800
To: "www-style@w3.org" <www-style@w3.org>
Message-ID: <56BAD562.1020302@inkedblade.net>
One major problem with "position: fixed" elements is that the content underneath
it is viewed as "scrolled into view" when in many cases it is in fact invisible
to the viewer. This interferes with paging actions, which no longer page by
screenful of visible content (which is the only really useful behavior).

Testcase:
http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=3875 [1]

The solution I have off the top of my head is to extend the definition of
'scroll-snap-padding' to address this case, since the use cases that drove
its addition are largely the same that result in this problem. That would
require renaming it to 'scroll-padding', and possibly adding an 'auto' value
that introduced padding based on any 'position: fixed' content applicable
to the viewport. (I haven't quite thought this through, though.)

I'm also concerned that "position: sticky" may have similar problems, though
perhaps that's solveable as a behavior constraint on UAs--to reduce the paging
size by the amount of sticky content--rather than something authors need to
address explicitly.

~fantasai

[1] data:text/html;charset=utf-8,<!DOCTYPE html>%0D%0A<p style%3D"position%3A fixed%3B top%3A 0%3B width%3A 100%25%3B 
height%3A 3em%3B background%3A orange%3B margin%3A 0%3B">Fixed Item<%2Fp>%0D%0AScrollable content 1<br>%0D%0AScrollable 
content 2<br>%0D%0AScrollable content 3<br>%0D%0AScrollable content 4<br>%0D%0AScrollable content 5<br>%0D%0AScrollable 
content 6<br>%0D%0AScrollable content 7<br>%0D%0AScrollable content 8<br>%0D%0AScrollable content 9<br>%0D%0AScrollable 
content 10<br>%0D%0AScrollable content 11<br>%0D%0AScrollable content 12<br>%0D%0AScrollable content 13<br>%0D%0AScrollable 
content 14<br>%0D%0AScrollable content 15<br>%0D%0AScrollable content 16<br>%0D%0AScrollable content 17<br>%0D%0AScrollable 
content 18<br>%0D%0AScrollable content 19<br>%0D%0AScrollable content 20
Received on Wednesday, 10 February 2016 06:15:35 UTC

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