<div class="search">
  <input id='search-btn' type='checkbox'/>
  <label for='search-btn'>显示搜索栏</label>
  <input id='search-bar' type='text' placeholder='w3cplus.com'/>
</div>

$btn-d: 5em;
$btn-r: .5*$btn-d;
$bar-w: 4*$btn-d;
$bar-h: .65*$btn-d;
$bar-r: .5*$bar-h;
$bar-c: #ffeacc;
$x: .5*($bar-w + $btn-r) - $btn-r;
$out-d: -3em;
$ico-d: .5*$bar-h;
$ico-f: .125;
$ico-w: $ico-f*$ico-d;

*, :before, :after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  font: inherit
}

html { overflow-x: hidden }

body {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 100vw;
  min-height: 100vh;
  background: #252525
}

[id='search-btn'] {
  position: absolute;
  left: -100vh;
  
  ~ * {
    --i: 0;
    --j: calc(1 - var(--i));
    transition: .65s;
  }
	
  &:checked ~ * { 
    --i: 1;
  }
}

.search {
  min-width: 400px;
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: row-reverse;
}

[for='search-btn'] {
  --hsl: 0, 0%, 0%;
  color: HSL(var(--hsl));
  
  display: block;
  overflow: hidden;
  width: $btn-d;
  height: $btn-d;
  border-radius: 50%;
  box-shadow: 0 0 1.5em rgba(#000, .4);
  text-indent: -100vw;
  cursor: pointer;
  position: relative;
  z-index: 1;
  
  transform: translate(calc(var(--j)*#{-$x})) rotate(45deg);
  

  $c0: #d9eb52; // green for collapsed state
  $c1: #dd1d6a; // pink for expanded state
  $h0: round(hue($c0)/1deg);
  $s0: round(saturation($c0));
  $l0: round(lightness($c0));
  $h1: round(hue($c1)/1deg);
  $s1: round(saturation($c1));
  $l1: round(lightness($c1));
  background: hsl(calc(var(--j)*#{$h0} + var(--i)*#{$h1}), 
                  calc(var(--j)*#{$s0} + var(--i)*#{$s1}), 
                  calc(var(--j)*#{$l0} + var(--i)*#{$l1}));
  

  &:before, &:after {
    position: absolute;
    top: 50%; 
    left: 50%;
    margin: -.5*$ico-d;
    width: $ico-d;
    height: $ico-d;
    transition: inherit;
    content: ''
  }
	
  &:before {
    margin-top: -.4*$ico-w;
    height: $ico-w;
    background: currentColor;
    transform-origin: 100% 50%;
    transform: 
      translate(calc(var(--j)*#{.25*$ico-d})) 
      scalex(calc(1 - var(--j)*.5));
  }
  
  &:after {
    box-shadow: inset 0 0 0 $ico-w currentcolor, inset 0 0 0 calc(var(--i)*#{.5*$ico-d}) HSLA(var(--hsl), var(--i));
    border-radius: calc(var(--j)*50%);
    transform: translate(calc(var(--j)*#{-.25*$ico-d})) 
            scalex(calc(var(--j) + var(--i)*#{$ico-f}));
  } 
}

[id='search-bar'] {
  border: none;
  padding: 0 1em;
  width: $bar-w;
  height: $bar-h;
  border-radius: $bar-r 0 0 $bar-r;
  background: #3f324d;
  color: #fff;
  font: 1em century gothic, verdana, arial, sans-serif;
  
  margin-right: -$btn-r;
  padding: 0 calc(#{$btn-r} + 1em) 0 1em;
  
 
  transform: translate(calc(var(--j)*#{$x}));
  clip-path: inset($out-d calc(var(--j)*#{$bar-w - $btn-r}) $out-d $out-d);
	
  &::placeholder {
    opacity: .5;
    color: inherit;
    font-size: .875em;
    letter-spacing: 1px;
    text-shadow: 0 0 1px, 0 0 2px
  }
	
  &:focus {
    outline: none;
    box-shadow: 0 0 1.5em $bar-c, 0 1.25em 1.5em rgba(#000, .2);
    background: $bar-c;
    color: #000;
  }
}