[css-contain] Consider 'discrete' Animation Type for 'content-visibility'

[zcorpan]

https://drafts.csswg.org/css-contain-2/#propdef-content-visibility defines the Animation Type as "not animatable". This makes it hard to transition between <div hidden=until-found> and not-hidden (and back), since the content disappears immediately.

The 'visibility' property has a 'discrete' Animation Type, which allows for transitions without the above problem.
https://drafts.csswg.org/css-display-3/#visibility

Compare:

• http://software.hixie.ch/utilities/js/live-dom-viewer/saved/10280
• http://software.hixie.ch/utilities/js/live-dom-viewer/saved/10282

Is there a reason for not using 'discrete' for 'content-visibility'?

[zcorpan]

cc @josepharhar

[josepharhar]

The second example which uses visibility:hidden both isn't searchable when hidden and exhibits the problem with chrome 102 which has hidden=until-found because it applies both visibility:hidden and content-visibility:hidden at the same time.
I made a new example which avoids using content-visibility completely to show what the desired animation looks like when clicking "show" and "hide": http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=10282

I agree this would be a good improvement for content-visibility. I'm not very familiar with animations so I don't know what the implementation would entail. @vmpstr

[zcorpan]

Oops, thanks @josepharhar. I accidentally tested in Chrome Stable where until-found is not supported.

[tabatkins]

Our general rule is that we don't let you animate things that change the box tree structure; this is somewhat inconvenient in general, and actually problematic in a cyclic sense if you can make the animating element stop having a box (that'll turn off the animation, which'll bring the box back, which'll restart the animation...).

But since content-visiblity just potentially affects the box-ness of its children, this might be acceptable.