incoming-view-transition.js

function hasPendingIncomingTransition() { }
function isRenderBlocked() { }
function continueIncomingTransition() { }

let hasSeenFold = false;
let lastFoldCheckTime = performance.now();
const FoldCheckThrolttle = 100;

function didSeeFold() {
  if (hasSeenFold || !hasPendingIncomingTransition())
    return;

  hasSeenFold = true;
  if (!isRenderBlocked())
    continueIncomingTransition();
}

function onRenderUnblocked() {
  if (hasSeenFold)
    continueIncomingTransition();
}

function onDOMContentLoaded() {
    didSeeFold();
}

function onParserYield() {
  if (!hasPendingIncomingTransition || hasSeenFold)
    return;

  const now = performance.now();
  if (now < (lastFoldCheckTime + FoldCheckThrolttle)
      return;
      
  computeStyle();
  layout();
  
  // In the future: consider adding something explicit rather than this heuristic
  if (elements.some(element => element.clientTop > toPixels("200vh"))) {
    didSeeFold();
  }
}