[css-view-transitions-2] Expose `activeViewTransition`

[bramus]

The spec refers to the active view transition which is added to the Document. However, this is not actually exposed to authors.

The React folks have asked me about this before in some conversations, and they are working around it by manually keeping track of the active view transition whenever startViewTransition is called.

I think it would make sense to add Document.activeViewTransition (and also Element.activeViewTransition for scoped transitions) for easier access.

[vmpstr]

I think this makes sense.

@nt1m @mattwoodrow @emilio WDYT?

[nt1m]

Seems fine to me and should be straightforward to do. I don't see any drawback to it

[emilio]

What's the deal with nested / etc view transitions? Do they reuse this slot? Are there multiple view transitions?

[emilio]

(Other than that I agree this seems fine)

[emilio]

Oh, I guess per the OP nested transitions would be in Element... I wonder if it's useful to expose whether there's any transition at the document level... Maybe not?

[bramus]

I wonder if it's useful to expose whether there's any transition at the document level... Maybe not?

With Scoped View Transitions multiple ones can run at the same time, so that would require a array then. Perhaps it could be:

• document.activeViewTransitions = array with all active view transitions.
• Element.activeViewTransition = the active view transition on that specific element

document.documentElement.activeViewTransition would return the active View Transition started on root, so that includes the one started using document.startViewTransition.

[vmpstr]

I'd want to limit this issue to just activeViewTransition on the element/document on which startViewTransition was called. I agree with @nt1m that it seems very straightforward to do.

If there's a need for activeViewTransitions array on the document that captures all transitions within this document, we can discuss separately (FWIW I think that would also be straightforward, but I want to make sure we see demand for that)

Proposal: add document.activeViewTransition that is the current ViewTransition and null otherwise. Add similar property for scoped view transitions, where each element also has an activeViewTransition field

[astearns]

The CSSWG will automatically accept this resolution one week from now if no objections are raised here. Anyone can add an emoji to this comment to express support. If you do not support this resolution, please add a new comment.

Proposed Resolution: Add activeViewTransition to Document and Element

[astearns]

RESOLVED: Add activeViewTransition to Document and Element