JavaScript during a transition
What needs the ViewTransition object itself: ready to animate a pseudo-element, skipTransition to call one off, the animations of the groups to stagger them. In this runtime a transition class can bring a hook: app.trans.name = (vt, cmds) => ...
Element.animate
A reveal from the press, animated from JavaScript once the transition is ready.
activeViewTransition
The same reveal with the engine as it is: a responder and document.activeViewTransition.
skipTransition
A hook that calls the transition off when the answer is for the page already open.
Staggered shuffle
Named items glide to their new place, one after the other, in their new order.
Slow motion in the header is JavaScript as well: a hook on forward and back sets the playbackRate of every view transition animation once it is ready.