<el-cascader-panel v-model="value" ref="panelRef" @expand-change="expandChange" />

const panelRef = ref(null)
panelRef.value.getBoundingClientRect()

报错：
panelRef.value.getBoundingClientRect is not a function

报错原因：
ref绑定在组件上，需要改成

panelRef.value.$el.getBoundingClientRect()