zotero/scss/elements/_itemPane.scss

item-pane {
	min-width: $min-width-item-pane + $width-sidenav;
	min-height: $min-width-item-pane + $width-sidenav;

	&[collapsed="true"] {
		min-width: $width-sidenav;
		min-height: $width-sidenav;
		max-width: $width-sidenav;

		&[view-type=note] {
			min-width: 0;
			width: 0px !important;
		}

		visibility: inherit;

		&.stacked {
			max-width: unset;
			max-height: $width-sidenav;

			&[view-type=note] {
				min-height: 0;
				height: 0px !important;
			}
		}

		#zotero-item-pane-content {
			visibility: collapse;
		}
	}

	&.stacked {
		flex-direction: column;
		height: 0px;

		#zotero-item-pane-content {
			width: 100%;
		}
	}
}