Layout Shift: How to prevent it ? (Cumulative Layout Shit / CLS)

About

A layout shift occurs when the rendering of the web page shifts / move element from their current position.

Causes

Image

For instance:

• without the knowledge of the target ratio (via the height and width attribute of an image, the browser can't reserve space for the image
• it download the image calculate the space needed for the image
• recalculate the layout
• and shifts element around to be able to put the image

That's what's called a layout shift

¹⁾

Solution:

• Set the correct image ratio on the image tag with the height and width attribute

Style

When you change a layout css property after page load (the first rendering), you will also create a layout shift.

Solution: If you want to change two layout property at the same time, you should use the window.requestAnimationFrame function to avoid a layout shift.

Example with a dynamic correction of a fixed top header such as the Bootstrap navbar where:

• we add the fixed-top class that set the position to fixed
• and correct it with a padding on the body.

window.requestAnimationFrame(function() {
    let fixedNavbar = document.querySelector(".navbar[data-type=\"fixed-top\"]")
    fixedNavbar.classList.add("fixed-top");
    let offsetHeight = fixedNavbar.offsetHeight;
    document.body.style.setProperty("padding-top",offsetHeight+"px");
});

Measurement

Occurences: Devtool

In the devtool, layout shift can be seen

• in the performance tab:
  • click the start and reload page (Ctrl+Shift+E)
  • stop it when the page has finish loaded
  • check the experience row.

• in the rendering tool where you can see the shifted region and and the web core vitals.

Cumulative layout shift

The Cumulative layout shift or CLS is a web core vital metrics that groups all layout shift during page load.

• The devtool can also add an overlay. See the web core vital for more information.

• A library can report back the cumulative layout shift (or CLS) measurement. See the example below

Calculation Example with Perfume

Example with perfume

• The element that we are going to shift

<button>Click to see the cumulative value</button>

• Simulate a layout shift

setTimeout(function(){
        let button = document.querySelector("button");
        button.style.display = "block";
        button.style.marginLeft= "auto";
        button.style.marginRight= "auto";
        button.style.width= "fit-content";
}
,2000);

• The engine

new Perfume({
  analyticsTracker: options => {
    const { metricName, data, eventProperties, navigatorInformation, vitalsScore, } = options;
    switch (metricName) {
      case 'cls':
        console.log('cumulativeLayoutShift value: '+data);
        break;
      case 'clsFinal':
        console.log('cumulativeLayoutShiftFinal: '+data);
        break;
    }
  },
});

• Output: