setup(_, ctx) {
 const originRef = ref<HTMLElement>();
 const panelRef = ref<HTMLElement>();
 const panelStyle = ref<CSSProperties>({ position: 'absolute' });
 // ...
 return () => (
  <>
   <div ref={originRef}>origin</div>

   <Teleport to="#cdk-overlay-anchor">
     <div style={{ position: 'fixed', top: 0, left: 0, right: 0, bottom: 0, height: '100vh', width: '100vw' pointerEvents: 'none'}}>
      <div ref={panelRef} style={panelStyle.value}>
       <div style={{height: '100px', width: '100px' border: '1px solid black'}} />
      </div>
     </div>
   </Teleport>
  </>
 );
}

const panelStyle = ref<CSSProperties>({ position: 'absolute' });

onMounted(() => {
 const origin = originRef.value;
 const panel = panelRef.value;
 if (!origin || !panel) {
  return ;
 }

 const calculate = () => {
  const rect = origin.getBoundingClientRect();
  // 原点为 origin 元素的底边中央正下方
  const originX = rect.left + (rect.width / 2);
  const originY = rect.bottom;

  // panel的坐标为到原点的偏移
  const panelRect = panel.getBoundingClientRect();
  const panelX = originX - panelRect.width / 2;
  const panelY = originY;

  // 设置 panel 数据，触发节点变更
  panelStyle.value.left = `${panelX}px`;
  panelStyle.value.top = `${panelY}px`;
 };
});

const origin$ = new MutationObserver(calculate);
origin$.observe(origin, {
 // 只需要拿到 attribute 的 style 的变化即可
 attributeFilter: ['style'],
});

const panel$ = new ResizeObserver(calculate);
panel$.observe(panel);

// dom销毁前取消监听
onBeforeUnmount(() => {
 origin$.disconnect();
 panel$.disconnect();
});

// 为了能够在滚动事件捕获前进行计算，带有滚动条的子元素也会因此触发计算
window.addEventListener('scroll', calculate, true);
window.addEventListener('resize', calculate);
最后，仍然要在销毁前取消事件。

// dom销毁前取消监听
onBeforeUnmount(() => {
 window.removeEventListener('scroll', calculate, true);
 window.removeEventListener('resize', calculate);
});