iter(g291): 下拉族七处弹层接上滚动跟随

packages/skin/ui/src/multiSelect.ts

@@ -1,7 +1,7 @@
 import { effect, h, signal } from '@ui-arch/frame';
 import type { Signal } from '@ui-arch/frame';
 import { bindOutside, pushOverlay } from './overlay.js';
-import { flip } from './position.js';
+import { flip, trackShift } from './position.js';
 import type { MultiSelectProps } from './types.js';
 
 /*** 多选下拉：标签式回显 + 可搜索弹层，value 数组信号双向绑定，到 max 未选项自动禁用 ***/ 
@@ -119,6 +119,15 @@ export function multiSelect(props: MultiSelectProps): HTMLElement {
     if (open()) releaseEsc ??= pushOverlay(() => { open(false); input.focus(); }); // 入栈逐层关闭
     else { releaseEsc?.(); releaseEsc = null; }
   });
-  effect(() => { if (open()) place(flip(box, pop, 'bottom')); });
+  let releaseShift: (() => void) | null = null; // 展开期间的位移跟随释放函数（滚动 / 缩放重算翻转）
+effect(() => {
+    if (!open()) { // 收起即摘监听：弹层不在，重算方位没有意义
+      releaseShift?.();
+      releaseShift = null;
+      return;
+    }
+    place(flip(box, pop, 'bottom')); // 打开先算一次：空间不足自动翻转
+    releaseShift ??= trackShift(() => place(flip(box, pop, 'bottom'))); // 展开期间跟着滚动重算：视口空间会变
+  });
   return root;
 }