<div class="funky funky--top">
  <span />
</div>

<div class="funky funky--right">
  <span />
</div>

<div class="funky funky--bottom">
  <span />
</div>

<div class="funky funky--left">
  <span />
</div>

:root {
  --border-width: 3rem;
  --squish-speed: 0.8s;
}

body {
  min-height: 100vh;
  margin: 0;
}

.funky {
  display: flex;
  position: absolute;
  background: orange;
}

.funky--top {
  top: 0;
  left: 0;
  z-index: 1;
  width: 100%;
}

.funky--right {
  right: 0;
  top: 0;
  height: 100vh;
  width: var(--border-width);
  flex-direction: column;
}

.funky--bottom {
  bottom: 0;
  left: 0;
  z-index: 1;
  width: 100%;
}

.funky--left {
  top: 0;
  left: 0;
  height: 100%;
  width: var(--border-width);
  flex-direction: column;
}

.funky > * {
  display: block;
  flex: 1;
  height: var(--border-width);
  background: orange;
  transition: all var(--squish-speed) ease-out;
  animation: grow var(--squish-speed) ease-out;
}

@keyframes grow {
  0%   { 
    flex-grow: 0.00001;
  }
  100% { 
    flex-grow: 1;
  }
}

document.body.style.backgroundColor = randomRgbaString(1);

var funkies = document.querySelectorAll('.funky');

for (const funky of funkies) {
  funky.addEventListener('mouseenter', function(event) {
     makeBar();
  })
}

function makeBar(target) {
  for (const funky of funkies) {
    var el = document.createElement('span');
    funky.appendChild(el);
    colorizeBars();
  }
}

function colorizeBars() {
  var bars = document.querySelectorAll('.funky > span');
  for (i = 0; i < bars.length; i++) {
    bars[i].style.backgroundColor = randomRgbaString(1);
  }
}

function randomRgbaString (alpha) {
  let r = Math.floor(Math.random() * 255)
  let g = Math.floor(Math.random() * 255)
  let b = Math.floor(Math.random() * 255)
  let a = alpha
  return `rgba(${r},${g},${b},${a})`
}