animationend event sampler

Layout read during startup

A custom element waits one microtask, attaches shadow DOM, reads scrollHeight, then sets data-state="opening" to start a 0s animation. This mirrors the original repro. The shadow CSS animates :host([data-state="opening"]) div. The handler sets data-state="done" when animationend fires.

The 0s animation gets its start time at kPaintClean, but finish events only dispatch on the next animation-frame timing update. If an on-demand partial lifecycle update lands in that gap and samples the animation as finished, affected builds drop it from servicing and the finish/animationend event is never dispatched. The timing is frame-scheduling dependent, so the miss rate varies by platform.