mastodon/app/javascript/styles/closed-social/timeline_comments.scss

.comments_timeline {
	max-height: 160px;
	overflow: hidden;
	-webkit-mask-image: linear-gradient(#1a1a1a,transparent);
	mask-image: linear-gradient(#1a1a1a,transparent);
	transform: scale(0.85);
	transform-origin: 80% 0%;
	padding-bottom: -32px;
	&:hover {
		max-height: none;
		-webkit-mask-image: none;
		mask-image: none;
	}
	&:active {
		max-height: none;
		-webkit-mask-image: none;
		mask-image: none;
	}
}