I absolutely love the design of the Sandwich site. Among many beautiful features are these headlines with rainbow underlines that move as you scroll. It’s not scroll-jacking — it’s just a minor design feature that uses scroll position to enact a little movement.
To draw the rainbows themselves, we could use a linear gradient with hard-stops, the same kinda concept as drawing stripes in CSS. That’s a big ol’ chunk of CSS, which is fine, but I see they’ve opted for a background-image instead. Here’s that as an SVG, which is 661 bytes (tiny tiny). We can make it look like an underline by setting the background-size to limit the height and position it along the bottom with background-position.
We’ll do it on an inline element so the underline breaks where the words break:
h1 {
span {
background-image: url(spectrum.svg);
background-repeat: repeat-x;
background-size: 100vw 0.2em;
background-position: left bottom 5px;
}
}
To animate it, we move the background-position-x. Not a particularly performant thing to animate, but we’re not really animating it anyway — it’s just moving based on scroll position. Rather than manually manipulate the background-position-x, we’ll set it with a custom property, then manipulate the custom property with JavaScript.
background-position-x: var(--scrollPos);
Updating that variable while the page scrolls is easy peezy:
window.addEventListener("scroll", e => {
let scrollTop = document.body.scrollTop ? document.body.scrollTop : document.documentElement.scrollTop;
let newPos = scrollTop "px";
document.documentElement.style.setProperty('--scrollPos', newPos);
});
Here it is working!
See that kinda janky line where I’m either using document.body or document.documentElement? That’s a stupid cross-browser thing where the “scrolling element” is different in Safari versus everything else.
While doing this I learned that you can use document.scrollingElement instead to avoid the pain there. I’ll leave a comment in the code about that, but leave the original line for posterity.
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