### Mouse cursor gradient tracking

A hover effect where the gradient follows the mouse cursor.

#### HTML

```html
<button class="mouse-cursor-gradient-tracking">
  <span>Hover me</span>
</button>
```

#### CSS

```css
.mouse-cursor-gradient-tracking {
  position: relative;
  background: #2379f7;
  padding: 0.5rem 1rem;
  font-size: 1.2rem;
  border: none;
  color: white;
  cursor: pointer;
  outline: none;
  overflow: hidden;
}

.mouse-cursor-gradient-tracking span {
  position: relative;
}

.mouse-cursor-gradient-tracking::before {
  --size: 0;
  content: '';
  position: absolute;
  left: var(--x);
  top: var(--y);
  width: var(--size);
  height: var(--size);
  background: radial-gradient(circle closest-side, pink, transparent);
  transform: translate(-50%, -50%);
  transition: width .2s ease, height .2s ease;
}

.mouse-cursor-gradient-tracking:hover::before {
  --size: 200px;
}
```

#### JavaScript

```js
var btn = document.querySelector('.mouse-cursor-gradient-tracking')
btn.onmousemove = function (e) {
  var x = e.pageX - btn.offsetLeft
  var y = e.pageY - btn.offsetTop
  btn.style.setProperty('--x', x + 'px')
  btn.style.setProperty('--y', y + 'px')
}
```

#### Demo

<div class="snippet-demo">
  <button class="snippet-demo__mouse-cursor-gradient-tracking">
    <span>Hover me</span>
  </button>
</div>

<style>
.snippet-demo__mouse-cursor-gradient-tracking {
  position: relative;
  background: #2379f7;
  padding: 0.5rem 1rem;
  font-size: 1.2rem;
  border: none;
  color: white;
  cursor: pointer;
  outline: none;
  overflow: hidden;
}

.snippet-demo__mouse-cursor-gradient-tracking span {
  position: relative;
}

.snippet-demo__mouse-cursor-gradient-tracking::before {
  --size: 0;
  content: '';
  position: absolute;
  left: var(--x);
  top: var(--y);
  width: var(--size);
  height: var(--size);
  background: radial-gradient(circle closest-side, aqua, rgba(0,255,255,0.0001));
  transform: translate(-50%, -50%);
  transition: width .2s ease, height .2s ease;
}

.snippet-demo__mouse-cursor-gradient-tracking:hover::before {
  --size: 200px;
}
</style>

<script>
(function () {
  var btn = document.querySelector('.snippet-demo__mouse-cursor-gradient-tracking')
  btn.onmousemove = function (e) {
    var x = e.pageX - btn.offsetLeft - btn.offsetParent.offsetLeft
    var y = e.pageY - btn.offsetTop - btn.offsetParent.offsetTop
    btn.style.setProperty('--x', x + 'px')
    btn.style.setProperty('--y', y + 'px')
  }
})()
</script>

#### Explanation

_TODO_

**Note!**

If the element's parent has a positioning context (`position: relative`), you will need to subtract
its offsets as well.

```js
var x = e.pageX - btn.offsetLeft - btn.offsetParent.offsetLeft
var y = e.pageY - btn.offsetTop - btn.offsetParent.offsetTop
```

#### Browser support
<div class="snippet__requires-javascript">Requires JavaScript</div>
<span class="snippet__support-note">⚠️ Requires JavaScript.</span>

* https://caniuse.com/#feat=css-variables