web-pps-fmp-polyfill.ts

​​let lastHeroElement;

// Simplified version of our TTFMP instrumentation
function recursiveCheckForFirstMeaningfulPaint() {
 const heroElement = document.getElementById("FMP-target");
 // To prevent against false positives during client-side route requests
 const isSameHeroElement = lastHeroElement === heroElement;
 const isImageHeroLoading =
   heroElement.tagName === "IMG" && !heroElement.complete;

 if (!heroElement || isSameHeroElement || isImageHeroLoading) {
   requestAnimationFrame(recursiveCheckForFirstMeaningfulPaint);
 } else {
   requestAnimationFrame(() => {
     lastHeroElement = heroElement;
     performance.measure("TTFMP");
   });
 }
}