<input id='search-btn' type='checkbox'/>
<label for='search-btn'>Show search bar</label>
<input id='search-bar' type='text' placeholder='Search...'/>

$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;
$out-d: -3em;
$x: .5*($bar-w + $btn-r) - $btn-r;
$ico-d: .5*$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;
	flex-direction: row-reverse;
	align-items: center;
	justify-content: center;
	margin: 0 auto;
	min-width: 400px; min-height: 100vh;
	background: #252525
}

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

[for='search-btn'] {
	$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));
	overflow: hidden;
	position: relative;
	z-index: 1;
	width: $btn-d; height: $btn-d;
	border-radius: 50%;
	transform: translate(calc(var(--j)*#{-$x})) rotate(45deg);
	box-shadow: 0 0 1.5em rgba(#000, .4);
	background: hsl(calc(var(--j)*#{$h0} + var(--i)*#{$h1}), 
                  calc(var(--j)*#{$s0} + var(--i)*#{$s1}), 
                  calc(var(--j)*#{$l0} + var(--i)*#{$l1}));
	--hsl: 0, 0%, 0%;
	color: HSL(var(--hsl));
	text-indent: -100vw;
	cursor: pointer;
	
	&: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: $ico-w;
		transform-origin: 100% 50%;
		transform: translate(calc(var(--j)*#{.25*$ico-d})) 
							 scalex(calc(1 - var(--j)*.5));
		background: currentcolor
	}
	
	&:after {
		border-radius: calc(var(--j)*50%);
		transform: translate(calc(var(--j)*#{-.25*$ico-d})) 
			scalex(calc(var(--j) + var(--i)*#{$ico-f}));
		box-shadow: inset 0 0 0 $ico-w currentcolor, 
			inset 0 0 0 calc(var(--i)*#{.5*$ico-d}) HSLA(var(--hsl), var(--i))
	}
}

[id='search-bar'] {
	border: none;
	margin-right: -$btn-r;
	padding: 0 calc(#{$btn-r} + 1em) 0 1em;
	width: $bar-w; height: $bar-h;
	border-radius: $bar-r 0 0 $bar-r;
	transform: translate(calc(var(--j)*#{$x}));
	background: #3f324d;
	color: #fff;
	font: 1em century gothic, verdana, arial, sans-serif;
	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 .75em 1.5em rgba(#000, .2);
		background: $bar-c;
		color: #000;
	}
}