demo · v140

Timing Diagram

When does the finished promise resolve? Before 140 it resolved within the rendering lifecycle — so any cleanup code that ran in .then() happened after the final frame was committed, producing a flicker. Post-140 it resolves before the cleanup frame.

pre-140 (flickers) post-140 (no flicker)

view transition animation frame commit cleanup script

document.startViewTransition(() => render()).finished.then(() => {
  // pre-140: this ran AFTER the final frame committed
  //          → one extra layout that often re-painted the page
  // post-140: runs BEFORE the cleanup frame
  //           → no flicker
  el.classList.remove("animating");
});

see also