[scroll-animations-1] Something is odd with the handling of `source` in the ScrollTimeline constructor

[birtles]

The ScrollTimeline constructor text has the following:

Creates a new ScrollTimeline object using the following procedure:

1. Let timeline be a new ScrollTimeline object.

2. Let source be the result corresponding to the first matching condition from below.
   If the source value of options is non-null,

   • Let source be source

   Otherwise (source is null):

   • Let source be the scrollingElement of the Document associated with the Window that is the current global object.

   Note: source may still be null after this step, e.g. if the Document has no scrollingElement.

3. Set the source of timeline to source.

Which is odd because source is not nullable in the ScrollTimeline spec. So how can we assign source if it is null?

Also, "Let source be source" is odd. Clearly it means the source member of options but it needs to say that.

[birtles]

If we decide that a source can be null then procedures like the one to resolve a container-based offset will need to be updated to say what to do in that case.

[birtles]

I notice that the procedure to resolve an element-based offset handles a null source so I suspect the IDL and "resolve a container-based offset" need to be updated (we should also check the other algorithms too, however).

[majido]

I guess the idea is that if the ScrollTimelineOptions may have a null source (which is the default value). And if that is given as input we resolve it to root scroller. So constructed timeline will never have a null source.

However thinking about this more, I wonder if we should allow a three state in our handling:

• Unspecified: Use document scroller e.g., new ScrollTimeline({}); /*or*/ new ScrollTimeline()
• Explicitly set to null: set source to to null e.g., new ScrollTimeline({source: null})
• Explicitly set to element: set source to element e.g., new ScrollTimeline({source: $div})

This is similar to how timeline is handler on Animation constructor.
I can make this change and clear up the wording mistake as well.

[birtles]

I guess the idea is that if the ScrollTimelineOptions may have a null source (which is the default value). And if that is given as input we resolve it to root scroller. So constructed timeline will never have a null source.

But the spec explicitly says that the Document can have no scrollingElement and hence source can be null?

However thinking about this more, I wonder if we should allow a three state in our handling:

• Unspecified: Use document scroller e.g., new ScrollTimeline({}); /*or*/ new ScrollTimeline()
• Explicitly set to null: set source to to null e.g., new ScrollTimeline({source: null})
• Explicitly set to element: set source to element e.g., new ScrollTimeline({source: $div})

That would solve this issue (once we also define the behavior when source is null, e.g. in the "resolve a container-based offset" algorithm). I'm not sure if it's useful to be able to do that, but maybe it's fine.

[majido]

Spec is fixed. I have filed a bug to fix Chromium implementation.