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[csswg-drafts] [css-scroll-snap] Visibility requirement makes x and y axis snapping dependent

From: Majid Valipour via GitHub <sysbot+gh@w3.org>
Date: Tue, 17 Jan 2017 17:11:23 +0000
To: public-css-archive@w3.org
Message-ID: <issues.opened-201348915-1484673082-sysbot+gh@w3.org>
majido has just created a new issue for 
https://github.com/w3c/csswg-drafts:

== [css-scroll-snap] Visibility requirement makes x and y axis 
snapping dependent ==
I think the current wording of [5.2.1. Scoping Valid Snap Positions to
 Visible Boxes](https://drafts.csswg.org/css-scroll-snap/#snap-scope) 
does not work well when there are visible snap alignments in both 
axis.

> a scroll container cannot be snapped to a snap area if no part of it
 is within the snapport.

Consider the following case where both snap alignment in x and y axis 
are valid by current definition. However if UA snaps to both of them 
in their respective axis the end result will be that neither will be 
visible which I believe is not the intended outcome of this section.

![2d-snapping-issue](https://cloud.githubusercontent.com/assets/944639/22029655/2e8cd8f8-dca9-11e6-9787-d4372cde8d1d.png)

Should the wording be updated to make it clear that the following snap
 position is not valid?

Regardless of the wording, I think the more fundamental issue is that 
each of the alignments is valid on its own axis but their combination 
is not valid. So to satisfy the visibility goal, the potential 
alignments on **both axis** should be considered **together**.

I am not sure what is the right solution/behavior here. Perhaps it 
should be left to UAs to decide how to satisfy this requirement. A 
naive solution will be to find the intended snap candidates and if 
they don't fit in snapport then drop one. But I suspect an optimal 
solution will require evaluation of all potential pairs which can be 
costly and complex. 

BTW, I also feel that the fact that this visibility requirement is 
coupling the snap position selection of x and y axis is somewhat in 
contrast with rest of the spec where it seems to wants to treat snap 
positions for each axis independently e.g. section 
[4.1.1](https://drafts.csswg.org/css-scroll-snap/#snap-axis): 
> snap positions are evaluated independently per axis


Please view or discuss this issue at 
https://github.com/w3c/csswg-drafts/issues/950 using your GitHub 
account
Received on Tuesday, 17 January 2017 17:11:29 UTC

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