layoutshifts_from_fonts.js

function getLastResource(time, regex) {
	let entries = performance.getEntriesByType('resource')
	let last = null;
    for(let i = 0; i < entries.length; i++) {
		let e = entries[i];
		if(regex && !e.name.match(regex)) continue;
		if(e.responseEnd < time) last = e;
	}
	return last;
}

let po = new PerformanceObserver((list) => {
    for (const entry of list.getEntries()) {
      //console.log(entry);
	  let val = entry.value;
	  let last = getLastResource(entry.startTime, /(woff)|(ttf)/);
	  if(!last) continue;
	  let diff = entry.startTime - last.responseEnd;
	  if(diff < 100) console.log(last.name + " was loaded " + diff + "ms before a layout shift with impact " + val);
    }
  });

  po.observe({type: 'layout-shift', buffered: true});