[scroll-animations] `ScrollTimeline` constructor has contradictory IDL and prose

[graouts]

The ScrollTimeline IDL reads as follows:

dictionary ScrollTimelineOptions {
  Element? source;
  ScrollAxis axis = "block";
};

[Exposed=Window]
interface ScrollTimeline : AnimationTimeline {
  constructor(optional ScrollTimelineOptions options = {});
};

And its specification text is:

1. Let timeline be the new ScrollTimeline object.
2. Set the source of timeline to:

• If the source member of options is present and not null,
  The source member of options.
• Otherwise,
  The scrollingElement of the Document associated with the Window that is the current global object.

To me, the IDL and the spec text are contradictory. Since the constructor is specified as taking optional ScrollTimelineOptions options = {} then I don't think we can distinguish between ScrollTimeline being constructed without an options parameter or with { source: null }. But I believe the spec text is actually trying to distinguish between three cases:

1. {} where the source would be set to document.scrollingElement,
2. { source: null } where the source would not be set,
3. { source: document.body } where the source would be set to document.body.

At least this is what is being tested by the WPT test scroll-animations/scroll-timelines/constructor.html.

[graouts]

Cc @flackr, @kevers-google and @andruud.

[cdumez]

Note that the Web IDL spec says:

If the type of an argument is a dictionary type or a union type that has a dictionary type as one of its flattened member types, and that dictionary type and its ancestors have no required members, and the argument is either the final argument or is followed only by optional arguments, then the argument must be specified as optional and have a default value provided.

[graouts]

To match the WPT test, I believe that:

1. the IDL should be set to constructor(optional ScrollTimelineOptions options),
2. the spec text should be updated to say "If the source member of options is present" and not "If the source member of options is present and not null".

That way implementations could distinguish between the three cases outlined above.

[graouts]

Upon closer inspection, the IDL constructor definition seems fine because ScrollTimelineOptions does not define source to default to null, so it is possible to distinguish between source being explicitly set to null and source not being explicitly set.

So I think the only required change is to update the spec text as I wrote just above. I'll make a PR to that effect.