.app:not(.presentation) .indent-mark .char-index:not(:nth-last-child(1)):not(:nth-last-child(2)) {
 	position: relative;
 }
.app:not(.presentation) .indent-mark .char-index:not(:nth-last-child(1)):not(:nth-last-child(2))::before {
	content: " ";
	position: absolute;
	left: 50%;
	margin: -12px -1.6px;
}
.app:not(.presentation) .indent-mark .char-index:nth-child(2n+1)::before {
	border-left: 2px solid #eee;  /* 色の指定 */
}
.app:not(.presentation) .indent-mark .char-index:nth-child(2n+2)::before {
	border-left: 2px solid #ddd;
}

複数行になる行で途切れるのってそういうもの？それとも僕の使い方が悪い？

あ、それが「自動改行や縦に長くなる記法（画像埋め込み等）を入れると線が途切れてしまっていたので、

 .indent-mark {
 	height: 100% !important;
 }
 .indent-mark .pad {
 	height: 100% !important;
 	overflow: unset !important;
 }
 .indent-mark span:nth-child(2n+2) .pad {
 	background: #f8f8f8;
 }