Faux nested matrix blocks (Craft 5 version)

craft-5-control-panel.css

.matrixblock[data-type="sectionBlock"] > .titlebar > .blocktype {
	color: var(--text-color);
	font-weight: 600;
}
.matrixblock[data-type="sectionBlock"] ~ .matrixblock:not([data-type="sectionBlock"]) {
	margin-left: 15px;
}

.matrixblock + .matrixblock[data-type="sectionBlock"] {
	margin-top: 30px;
}

.matrixblock[data-type="sectionBlock"] ~ .matrixblock:not([data-type="sectionBlock"])::before {
	background: var(--gray-100);
	bottom:0;
	content: "";
	left:-17px;
	position: absolute;
	top:0;
	width: 2px;
}

.matrixblock[data-type="sectionBlock"] ~ .matrixblock:not([data-type="sectionBlock"])::before {
	top:-15px;
}

.matrixblock:has(+ .matrixblock[data-type="sectionBlock"])::before,
.matrixblock:not([data-type="sectionBlock"]):last-child::before {
	bottom: 50% !important;
}

.matrixblock[data-type="sectionBlock"] ~ .matrixblock:not([data-type="sectionBlock"])::after {
	background: var(--gray-100);
	content: "";
	height: 2px;
	left:-17px;
	position: absolute;
	top:50%;
	transform: translateY(-50%);
	width: 16px;
}