fix(components): [table] fix absence of theadRef bug (#21454)

fix(components): [TableHeader] fix absence of theadRef bug

packages/components/table/src/table-header/index.ts

@@ -4,6 +4,7 @@ import {
   h,
   inject,
   nextTick,
+  onBeforeUnmount,
   onMounted,
   reactive,
   ref,
@@ -80,8 +81,9 @@ export default defineComponent({
     const saveIndexSelection = reactive(new Map())
     const theadRef = ref()
 
+    let delayId: ReturnType<typeof setTimeout> | undefined
     const updateFixedColumnStyle = () => {
-      setTimeout(() => {
+      delayId = setTimeout(() => {
         if (saveIndexSelection.size > 0) {
           saveIndexSelection.forEach((column, key) => {
             const el = theadRef.value.querySelector(
@@ -98,6 +100,12 @@ export default defineComponent({
     }
 
     watch(saveIndexSelection, updateFixedColumnStyle)
+    onBeforeUnmount(() => {
+      if (delayId) {
+        clearTimeout(delayId)
+        delayId = undefined
+      }
+    })
 
     onMounted(async () => {
       // Need double await, because updateColumns is executed after nextTick for now