[css-view-transitions] State management seems broken in the spec

[emilio]

Per spec, this snippet calls the transition callback twice:

document.startViewTransition(async function() {
  console.log("cb");
  document.startViewTransition(() => {});
});

This is in fact what happens in Safari TP, and in my experimental Firefox implementation.

This is because call the update callback says:

• Otherwise, set callbackPromise to the result of invoking transition’s update callback.
• If transition’s phase is not "done", then set transition’s phase to "update-callback-called".

But that's already too late, because if something (like startViewTransition) skips the transition, the state is not "update-callback-called".

It seems for this specific case, just swapping those two steps should work. But might be worth looking at similar things in the spec.

cc @khushalsagar @vmpstr @mattwoodrow @nt1m

[emilio]

cc @noamr too.

[noamr]

Good catch, I think swapping them should be sufficient, as the previous steps in call the update callback are just setup and don't have any effect.

[emilio]

Yeah at first I was more concerned about this being a general problem, but this is the only place where I found the issue so far.

[nt1m]

I'm fixing this in WebKit with a WPT: WebKit/WebKit#33091

I'm not sure the exact wording of the current spec PR is right though (I commented there already)