scrollable tabbox tabs breaking in classic themes

<zk>
		<tabbox width="500px">
			<tabs id="tabs"/>
			<tabpanels height="100px"/>
		</tabbox>
		<button label="Add Tab">
			<attribute name="onClick"><![CDATA[
				Tab tb = new Tab();
				tb.setLabel("Tab panel test test test test");
				tb.setClosable(true);
				tb.setSelected(true);
				tabs.appendChild(tb);
			]]></attribute>
		</button>
</zk>

		.z-tabs-content {
			display: flex;
		}
		.z-tabs::before {
			content: ' ';
			display: block;
			position: absolute;
			bottom: 0px;
			height: 1px;
			border-bottom: 1px solid #cfcfcf;
			width: 10000px;
		}