Layout Shift: How to prevent it ?

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

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

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Solution:

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

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");
});

In the devtool, layout shift can be seen

• in the performance tab

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