zotero/scss/components/_splitter.scss

splitter {
	appearance: none;
	z-index: 1;
	position: relative;
	background: transparent;
	border: none;

	--draggable-size: 5px;
	
	@include compact {
		--draggable-size: 5px;

		&[state="collapsed"] {
			--draggable-size: 8px;
		}
	}
	@include comfortable {
		--draggable-size: 8px;

		&[state="collapsed"] {
			--draggable-size: 10px;
		}
	}

	&[orient=vertical] {
		min-height: var(--draggable-size);

		&[collapse="before"],
		&[substate="before"] {
			border-top: var(--material-border-quarternary);
			margin-bottom: calc(1px - var(--draggable-size));
			margin-top: -1px;
			&[state="collapsed"] {
				border-bottom: var(--material-border-quarternary);
				border-top: 0;
				margin-bottom: 0;
				margin-top: 0;
			}
		}

		&[collapse="after"],
		&[substate="after"] {
			border-bottom: var(--material-border-quarternary);
			margin-top: calc(1px - var(--draggable-size));
			margin-bottom: -1px;
			&[state="collapsed"] {
				border-top: var(--material-border-quarternary);
				border-bottom: 0;
				margin-top: 0;
				margin-bottom: 0;
			}
		}
	}

	&:not([orient=vertical]) {
		min-width: var(--draggable-size);

		&[collapse="before"],
		&[substate="before"],
		&[collapse="after"]:-moz-locale-dir(rtl),
		&[substate="after"]:-moz-locale-dir(rtl) {
			border-left: var(--material-border-quarternary);
			margin-right: calc(1px - var(--draggable-size));
			margin-left: -1px;
			&[state="collapsed"] {
				border-right: var(--material-border-quarternary);
				border-left: 0;
				margin-right: 0;
				margin-left: 0;
			}
		}

		&[collapse="after"],
		&[substate="after"],
		&[collapse="before"]:-moz-locale-dir(rtl),
		&[substate="before"]:-moz-locale-dir(rtl) {
			border-right: var(--material-border-quarternary);
			margin-left: calc(1px - var(--draggable-size));
			margin-right: -1px;
			&[state="collapsed"] {
				border-left: var(--material-border-quarternary);
				border-right: 0;
				margin-left: 0;
				margin-right: 0;
			}
		}
	}
}

#zotero-items-splitter,
#zotero-context-splitter {
	&[state="collapsed"] {
		--draggable-size: 1px;
		border: 0;
		margin-left: -1px;
	}
}