Gadget-expand.css

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/*
todo: proper themed colors, credits
*/

.content {
	transition: max-width ease-in-out 0.5s;
}

.content-expanded {
	max-width:100%;
}

.gadget-expand-container {
	position: absolute;
	top: 65px;
	left: -12px;
	height: 100%;
}

.gadget-expand-button {
	position: sticky;
	background: firebrick;
	width: 34px;
	height: 34px;
	top: 125px;
	border-radius: 35px;
	color: white;
	display: flex;
	justify-content: center;
	align-content: center;
	box-shadow: 0 3px 12px 0 rgba(0,0,0,.3);
	cursor: pointer;
}

.gadget-expand-button:hover {
	color: lightgray !important;
}

MediaWiki

Gadget-expand.css