style.css

* {
	box-sizing: border-box;
}

html {
	width: 100%;
	height: 100%;
}

body {
	width: 100%;
	height: 100%;
}

body {
	margin: 0;
}

.explorer {
	width: 100%;
	height: 100%;
	display: flex;
	flex-direction: column;
}

.explorer-header {
	flex: 0 0 auto;
	border: 2px solid gray;
}

.explorer-body {
	flex-grow: 1;
	display: flex;
	border: 2px solid gray;
	border-top: none;
}

.parent-container {
	background-color: pink;
	padding: 10px;
	overflow: hidden;
}

.parent-item, .sibling-item, .child-item {
	display: inline-block;
	height: 200px;
	width: 200px;
	background-color: lightgray;
	opacity: .7;
	padding: 5px;
	border: 2px solid darkgray;
	border-radius: 10px;
	overflow: hidden;
}

.parent-item.current {
	background-color: limegreen;
}

.parent-item:not(:last-of-type) {
	margin-right: 10px;
}

.sibling-container {
	border-right: 2px solid gray;
	background-color: lightblue;
	padding: 10px;
	overflow: hidden;
}

.sibling-item {
	display: block;
}

.sibling-item.current {
	background-color: limegreen;
}

.sibling-item:not(:last-of-type) {
	margin-bottom: 10px;
}

.content-container {
	flex-grow: 1;
	background-color: #FF7256;
}

.square {
	position: relative;
}

.square:before {
	content: "";
	display: block;
	padding-top: 100%;
}

.square > * {
	position: absolute;
	top: 0;
	left: 0;
	bottom: 0;
	right: 0;
}

.content-container {
	padding: 10px;
}

.child-item:not(:last-of-type) {
	margin-right: 10px;
}