zotero/scss/elements/_itemPane.scss

item-pane {
	flex-grow: 0;
	flex-shrink: 1;
	min-width: var(--width-available-item-pane, $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;

		visibility: inherit;

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

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

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

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