Bookmarklet for showing Cumulative Layout Shift

CumulativeLayoutShiftBookmarklet.js

/**
 * A bookmarklet for viewing shifted elements while debugging
 * Cumulative Layout Shift (web.dev/cls). Works in Chrome 84+
 * Shows the previous position of shifted elements in yellow,
 * and the new position in red.
 *
 * To install:
 * 1. Copy the code starting from the line beginning `javascript:`
 * 2. Add a new bookmark in Chrome, and paste the code in as the URL.
 **/
javascript:(function(){
  try {
    const canvas = document.createElement('canvas');
    canvas.style.width = '100%';
    canvas.style.height = '100%';
    canvas.width = window.innerWidth;
    canvas.height = window.innerHeight;
    canvas.style.position = 'absolute';
    canvas.style.left = 0;
    canvas.style.top = 0;
    canvas.style.zIndex = 100000;
    canvas.style.pointerEvents = 'none';
    document.body.appendChild(canvas);
    const context = canvas.getContext('2d');
    const po = new PerformanceObserver((list) => {
      //context.clearRect(0, 0, canvas.width, canvas.height);
      for (const entry of list.getEntries()) {
        canvas.style.transition = 'opacity 0s';
        canvas.style.opacity = 1;
        for (const source of entry.sources) {
          context.strokeStyle = 'yellow';
          context.strokeRect(source.previousRect.x,
                             source.previousRect.y,
                             source.previousRect.width,
                             source.previousRect.height);
          context.strokeStyle = 'red';
          context.strokeRect(source.currentRect.x,
                             source.currentRect.y,
                             source.currentRect.width,
                             source.currentRect.height);
          console.log(source);
        }
        setTimeout(function() {
          canvas.style.transition = 'opacity 2s';
          canvas.style.opacity = 0;
        }, 1000);
      }
    });

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