fix(DropdownMenu): fix dropdown-item position when expanded by default (#11640)

* fix(DropdownMenu): fix dropdown-item position when expanded by default

* chore: delete redundant code

packages/vant/src/dropdown-item/DropdownItem.tsx

@@ -100,6 +100,7 @@ export default defineComponent({
       state.transition = !options.immediate;
 
       if (show) {
+        parent.updateOffset();
         state.showWrapper = true;
       }
     };

packages/vant/src/dropdown-menu/DropdownMenu.tsx

@@ -103,7 +103,6 @@ export default defineComponent({
     const toggleItem = (active: number) => {
       children.forEach((item, index) => {
         if (index === active) {
-          updateOffset();
           item.toggle();
         } else if (item.state.showPopup) {
           item.toggle(false, { immediate: true });
@@ -143,7 +142,7 @@ export default defineComponent({
       );
     };
 
-    linkChildren({ id, props, offset });
+    linkChildren({ id, props, offset, updateOffset });
     useClickAway(root, onClickAway);
     useEventListener('scroll', onScroll, {
       target: scrollParent,

packages/vant/src/dropdown-menu/types.ts

@@ -7,6 +7,7 @@ export type DropdownMenuProvide = {
   id: string;
   props: DropdownMenuProps;
   offset: Ref<number>;
+  updateOffset: () => void;
 };
 
 export type DropdownMenuThemeVars = {