Web Designing

You can increase the background-size (500px in this example) to increase transition smoothing.

const e = document.getElementsByClassName('scan')[0]; const hidden = document.getElementsByClassName('hidden')[0]; document.onmousemove = function(event) { e.style.left = `${event.clientX}px`; // ↓ background width (500px) / 2 hidden.style.setProperty("--pos", `${event.clientX - 250}px`); }; * { margin: 0; padding: 0; } html, body { width: 100%; min-height: 100vh; overflow-x: hidden; display: flex; } .banner { width: 100vw; height: 100vh; display: flex; flex-grow: 1; flex-direction: row; align-items: center; background-color: #031321; } .banner .scan { width: 7px; height: 80%; position: absolute; left: 30px; z-index: 3; transition: left 50ms ease-out 0s; border-radius: 15px; background-color: #fff; box-shadow: 0 0 15px 5px #fff, /* inner white */ 0 0 35px 15px #008cff, /* inner blue */ 0 0 350px 20px #0ff; /* outer cyan */ } .banner .description { width: 100%; color: white; font-size: 3em; text-align: center; -webkit-user-select: none; -ms-user-select: none; user-select: none; } .hidden { background: radial-gradient(dodgerblue 10%, #031321 50%) var(--pos) 50% / 500px 500px no-repeat fixed; -webkit-background-clip: text; -webkit-text-fill-color: transparent; } <div class="banner"> <div class="scan"></div> <div class="description"> Just <span class="hidden">hidden</span> something </div> </div>