[scroll-animations-1] ViewTimelineOptions dictionary should accept subject

[flackr]

The proposed rewrite in #6674 adds a ViewTimeline, which produces a time value based on an element's position within the scroller. The dictionary passed to the ViewTimeline constructor is defined as:

dictionary ViewTimelineOptions {
  Element source;
  ScrollDirection axis = "block";
};

Given that ViewTimeline extends ScrollTimeline I assume the source is the scrolling element whose scroll position is to be observed. I think the dictionary should have a subject member, to set the readonly subject attribute on the ViewTimeline.

[kevers-google]

Agreed on having subject as an option; however, it would allow us to get into an inconsistent state if subject and source are both set, since the source should be the nearest scrolling ancestor to the subject, which might not match the specified source. Though we could make the default source the nearest scrolling ancestor and allow setting a different source.

[fantasai]

I feel like source here was supposed to be subject and this is just a (really stupid) error on my part. The source can be derived from the subject, but you can't create a ViewTimeline without specifying a subject!

(That said, I'm open to better names for these, I feel like they're so abstract it's confusing.)

[css-meeting-bot]

The CSS Working Group just discussed [scroll-animations-1] ViewTimelineOptions dictionary should accept subject.

The full IRC log of that discussion

<dael> Topic: [scroll-animations-1] ViewTimelineOptions dictionary should accept subject
<dael> github: https://github.com//issues/7157
<dael> fantasai: I think this was a copy/paste error where I forgot to switch source to subject. I don't think need to discuss unless someone thinks it's correct

[flackr]

Looks like this landed in cd2ad9b