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Re: [csswg-drafts] [css-contain-2] content-visibility should pause css animations in subtree (#5611)

From: vmpstr via GitHub <sysbot+gh@w3.org>
Date: Wed, 18 Nov 2020 16:12:57 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-729784858-1605715976-sysbot+gh@w3.org>
The goal of this proposal is to not tick animations when they are under a content-visibility hidden subtree in order to avoid doing work in such subtrees. AFAIK the only observable effect of this is that animations would not start/end in a hidden subtree unless the update is somehow forced.

Is it feasible for us to add leeway for the implementations by saying something like "the user-agent _may_ not process animations updates while the contents are under a hidden subtree". I think that would leave room for us to do the updates  when updates are forced but also allow the optimization? I know this is a weaker language but it seems hard to specify exact behavior under all situations.

One other issue I ran into is that it's not trivial for us to do these updates when we getAnimations with subtree: true. This is because the subtree may or may not have content-visibility elements in it. Without first finding that out, we can't force the update on those elements easily. This is a bit of an implementation difficulty in Blink, but I think depending on the spec language we decide to use here, it may be necessary.

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