bootstrap-less-port/less/_transitions.less

A Less port of Bootstrap v4

.fade {
	#transition(@transition-fade);

	&:not(.show) {
		opacity: 0;
	}
}

.collapse {
	&:not(.show) {
		display: none;
	}
}

.collapsing {
	position: relative;
	height: 0;
	overflow: hidden;
	#transition(@transition-collapse);
}