# Pastebin kYO310cz
let gCallbacks = {
  beforeLayout: [],
  afterLayout: [],
};

function runCallbacksForThisFrame() {
  for (const callback of gCallbacks.beforeLayout) {
    callback();
  }
  document.body.offsetTop; // Trigger layout
  for (const callback of gCallbacks.afterLayout) {
    callback();
  }
}

function promiseLayoutFlushed(callback) {
  // Assume all the promise stuff...
  if (gCallbacks.beforeLayout.length === 0 && gCallbacks.afterLayout.length === 0) {
    requestAnimationFrame(runCallbacksForThisFrame);
  }

  gCallbacksForThisFrame.afterLayout.push(callback);
}

function promiseLayoutFlushed(callback) {
  if (gCallbacks.beforeLayout.length === 0 && gCallbacks.afterLayout.length === 0) {
    requestAnimationFrame(runCallbacksForThisFrame);
  }

  gCallbacksForThisFrame.beforeLayout.push(callback);
}