zotero/scss/components/_mainWindow.scss

:is(panel)::part(content) {
	--panel-background: var(--material-toolbar);
}

#main-window {
	min-width: $min-width-collections-pane + $min-width-items-pane + $min-width-item-pane + $width-sidenav;
	min-height: 300px;
}

#zotero-pane-stack > hbox {
	// Let window chrome, tabs, and toolbar shrink even when sidebars overflow
	min-width: 0;
}