fix(dropdown): 展开层改 Portal + fixed 定位，逃出弹窗 overflow 裁剪

弹窗（.modal 带 overflow-y:auto）里的下拉展开层是 absolute 定位，
会被弹窗滚动盒裁掉下半截（Git 同步设置里实测被遮挡）。展开层改经
Portal 挂到 body、按触发钮 rect 用 fixed 定位：下方空间不足时
向上翻，maxHeight 按可用空间夹取；窗口 resize 时关闭。
useDismiss 泛化为可传多个 ref（触发器与 Portal 菜单两棵子树都算内）。
CSS 相应去掉 .dropdown-menu 的 absolute 定位（几何改由内联给出）。
验证：vitest 124+2（新增翻转几何断言：贴底触发钮翻上展开、
Portal 落点不在组件容器内）；无头 Chromium 实测弹窗内下拉菜单
完整落在视口内、8 项全显、已逃出弹窗 DOM。

src/components/Dropdown.tsx

@@ -1,11 +1,21 @@
 // 自绘下拉（替代原生 select：原生展开层是系统样式，与应用风格脱节）。
 // 外观对齐 .input；展开层沿用应用菜单语言（面板底、6px 圆角、悬停 --hover、
 // 选中 --accent-weak）；点别处/Esc 关闭走统一的 useDismiss；支持上下键选行。
-import { useCallback, useEffect, useRef, useState } from 'react'
+// 展开层经 Portal 挂到 body 并用 fixed 定位（按触发钮 rect 计算、下方空间
+// 不足时向上翻）：弹窗等带 overflow 的容器里 absolute 定位会被滚动盒裁掉
+// （Git 同步设置里实测被遮挡），Portal + fixed 才能真正逃出裁剪。
+import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
+import { createPortal } from 'react-dom'
 import { useDismiss } from '../hooks/useDismiss'
 
 export interface DropdownOption { value: string; label: string }
 
+interface MenuPos { left: number; top: number; width: number; maxHeight: number }
+
+const ITEM_H = 31
+const MENU_PAD = 8
+const MAX_H = 264
+
 export function Dropdown({ value, options, onChange, disabled, ariaLabel }: {
   value: string
   options: DropdownOption[]
@@ -14,13 +24,35 @@ export function Dropdown({ value, options, onChange, disabled, ariaLabel }: {
   ariaLabel?: string
 }) {
   const [open, setOpen] = useState(false)
+  const [pos, setPos] = useState<MenuPos | null>(null)
   const ref = useRef<HTMLDivElement>(null)
+  const btnRef = useRef<HTMLButtonElement>(null)
   const menuRef = useRef<HTMLDivElement>(null)
   const close = useCallback(() => setOpen(false), [])
-  useDismiss(ref, open, close)
+  const dismissRefs = useMemo(() => [ref, menuRef], [])
+  useDismiss(dismissRefs, open, close)
 
   const current = options.find((o) => o.value === value) ?? options[0]
 
+  // 打开时按触发钮位置算展开层几何：优先向下，下方放不下且上方更宽裕时翻上去
+  const openMenu = useCallback(() => {
+    const rect = btnRef.current?.getBoundingClientRect()
+    if (!rect) { setOpen(true); return }
+    const est = Math.min(MAX_H, options.length * ITEM_H + MENU_PAD)
+    const spaceBelow = window.innerHeight - rect.bottom - 8
+    const spaceAbove = rect.top - 8
+    const above = spaceBelow < est && spaceAbove > spaceBelow
+    const maxHeight = Math.max(80, Math.min(MAX_H, above ? spaceAbove : spaceBelow))
+    const height = Math.min(est, maxHeight)
+    setPos({
+      left: Math.max(4, Math.min(rect.left, window.innerWidth - rect.width - 4)),
+      top: above ? Math.max(4, rect.top - height - 4) : rect.bottom + 4,
+      width: rect.width,
+      maxHeight,
+    })
+    setOpen(true)
+  }, [options.length])
+
   // 打开时把焦点放到选中项，便于直接上下键选行
   useEffect(() => {
     if (!open) return
@@ -29,6 +61,14 @@ export function Dropdown({ value, options, onChange, disabled, ariaLabel }: {
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [open ])
 
+  // 窗口尺寸变化时展开层位置失效，直接关闭（与菜单类组件惯例一致）
+  useEffect(() => {
+    if (!open) return
+    const onResize = () => setOpen(false)
+    window.addEventListener('resize', onResize)
+    return () => window.removeEventListener('resize', onResize)
+  }, [open ])
+
   const pick = (v: string) => { setOpen(false); onChange(v) }
 
   const onMenuKey = (e: React.KeyboardEvent) => {
@@ -46,17 +86,23 @@ export function Dropdown({ value, options, onChange, disabled, ariaLabel }: {
     <div className="dropdown" ref={ref}>
       <button
         type="button"
+        ref={btnRef}
         className="dropdown-button"
         disabled={disabled}
         aria-label={ariaLabel}
         aria-expanded={open}
-        onClick={() => setOpen((o) => !o)}
+        onClick={() => { if (open) setOpen(false); else openMenu() }}
       >
         <span className="dropdown-label">{current?.label ?? ''}</span>
         <svg width="10" height="10" viewBox="0 0 10 10" aria-hidden="true"><path d="M2 3.5 L5 6.5 L8 3.5" fill="none" stroke="currentColor" strokeWidth="1.3" strokeLinecap="round" strokeLinejoin="round" /></svg>
       </button>
-      {open && (
+      {open && pos && createPortal(
-        <div className="dropdown-menu" ref={menuRef} onKeyDown={onMenuKey}>
+        <div
+          className="dropdown-menu"
+          ref={menuRef}
+          onKeyDown={onMenuKey}
+          style={{ position: 'fixed', left: pos.left, top: pos.top, width: pos.width, maxHeight: pos.maxHeight, zIndex: 200 }}
+        >
           {options.map((o) => (
             <button
               key={o.value}
@@ -67,7 +113,8 @@ export function Dropdown({ value, options, onChange, disabled, ariaLabel }: {
               {o.label}
             </button>
           ))}
-        </div>
+        </div>,
+        document.body,
       )}
     </div>
   )

src/hooks/useDismiss.ts

@@ -5,12 +5,22 @@ import { useEffect, type RefObject } from 'react'
  * 按 Esc、窗口失焦、或菜单外发生滚动时关闭。
  * 指针监听用捕获阶段：先于 React 的 contextmenu 处理执行，
  * 这样在另一个标签上右键时是「先关旧菜单、再开新菜单」，不会互相冲掉。
+ * refs 可传多个：菜单经 Portal 渲染到 body 时（如下拉展开层），触发器与
+ * 菜单是两个分离的 DOM 子树，两边都算「内」。
  */
-export function useDismiss(ref: RefObject<HTMLElement | null>, active: boolean, onClose: () => void) {
+export function useDismiss(
+  refs: RefObject<HTMLElement | null> | RefObject<HTMLElement | null>[],
+  active: boolean,
+  onClose: () => void,
+) {
   useEffect(() => {
     if (!active) return
-    const outside = (target: EventTarget | null) =>
+    const list = Array.isArray(refs) ? refs : [refs]
-      ref.current && target instanceof Node && !ref.current.contains(target)
+    const outside = (target: EventTarget | null) => {
+      if (!(target instanceof Node)) return false
+      const containers = list.map((r) => r.current).filter(Boolean) as HTMLElement[]
+      return containers.length > 0 && containers.every((el) => !el.contains(target))
+    }
     const onPointer = (e: MouseEvent) => { if (outside(e.target)) onClose() }
     const onKey = (e: KeyboardEvent) => { if (e.key === 'Escape') onClose() }
     const onScroll = (e: Event) => { if (outside(e.target)) onClose() }
@@ -26,5 +36,5 @@ export function useDismiss(ref: RefObject<HTMLElement | null>, active: boolean,
       document.removeEventListener('scroll', onScroll, true)
       window.removeEventListener('blur', onClose)
     }
-  }, [ref, active, onClose])
+  }, [refs, active, onClose])
 }

src/styles.css

@@ -358,7 +358,8 @@ kbd { background: var(--code-bg); border: 1px solid var(--border); border-radius
 .dropdown-button:disabled { opacity: 0.5; cursor: default; }
 .dropdown-button svg { flex: none; color: var(--text-dim); }
 .dropdown-label { overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
-.dropdown-menu { position: absolute; top: calc(100% + 4px); left: 0; right: 0; z-index: 60; background: var(--bg-editor); border: 1px solid var(--border); border-radius: 8px; padding: 4px; box-shadow: 0 10px 28px rgba(0,0,0,0.18); display: flex; flex-direction: column; max-height: 264px; overflow-y: auto; }
+/* 展开层几何（fixed 定位/宽/最大高）由 Dropdown 组件内联给出：Portal 到 body 逃出弹窗等 overflow 容器的裁剪 */
+.dropdown-menu { background: var(--bg-editor); border: 1px solid var(--border); border-radius: 8px; padding: 4px; box-shadow: 0 10px 28px rgba(0,0,0,0.18); display: flex; flex-direction: column; overflow-y: auto; }
 .dropdown-menu button { border: 0; background: transparent; color: var(--text); padding: 6px 9px; border-radius: 5px; font-size: 13px; font-family: inherit; text-align: left; cursor: pointer; }
 .dropdown-menu button:hover, .dropdown-menu button:focus-visible { background: var(--hover); outline: none; }
 .dropdown-menu button.selected { background: var(--accent-weak); }

tests/dropdown.test.tsx

@@ -22,18 +22,44 @@ describe('Dropdown', () => {
     const button = container.querySelector('.dropdown-button') as HTMLButtonElement
     expect(button.textContent).toContain('GitHub')
     await act(async () => { button.dispatchEvent(new MouseEvent('click', { bubbles: true })) })
-    const menu = container.querySelector('.dropdown-menu')
+    const menu = document.querySelector('.dropdown-menu')
     expect(menu).not.toBeNull()
     expect(menu?.querySelector('button.selected')?.textContent).toBe('GitHub')
     const notion = [...menu!.querySelectorAll('button')].find((b) => b.textContent === 'Notion')
     await act(async () => { notion?.dispatchEvent(new MouseEvent('click', { bubbles: true })) })
     expect(onChange).toHaveBeenCalledWith('notion')
-    expect(container.querySelector('.dropdown-menu')).toBeNull()
+    expect(document.querySelector('.dropdown-menu')).toBeNull()
     // 再次展开，点别处关闭
     await act(async () => { button.dispatchEvent(new MouseEvent('click', { bubbles: true })) })
-    expect(container.querySelector('.dropdown-menu')).not.toBeNull()
+    expect(document.querySelector('.dropdown-menu')).not.toBeNull()
     await act(async () => { document.body.dispatchEvent(new MouseEvent('mousedown', { bubbles: true })) })
+    expect(document.querySelector('.dropdown-menu')).toBeNull()
+    await act(async () => { root.unmount() })
+    container.remove()
+  })
+
+  it('展开层经 Portal 挂到 body；触发钮靠近窗口底部时向上翻且不溢出', async () => {
+    const container = document.createElement('div')
+    document.body.appendChild(container)
+    const root = createRoot(container)
+    await act(async () => {
+      root.render(<Dropdown value="github" options={OPTIONS} onChange={() => undefined} />)
+    })
+    const button = container.querySelector('.dropdown-button') as HTMLButtonElement
+    // 模拟触发钮贴近窗口底部（jsdom 默认视口高 768）：下方只剩 30px
+    button.getBoundingClientRect = () => ({
+      top: 700, bottom: 730, left: 100, right: 300, width: 200, height: 30, x: 100, y: 700, toJSON: () => ({}),
+    }) as DOMRect
+    await act(async () => { button.dispatchEvent(new MouseEvent('click', { bubbles: true })) })
+    // Portal：菜单不在组件容器内，而是 body 直属
     expect(container.querySelector('.dropdown-menu')).toBeNull()
+    const menu = document.querySelector('.dropdown-menu') as HTMLElement | null
+    expect(menu).not.toBeNull()
+    expect(menu!.style.position).toBe('fixed')
+    // 3 项估算高 101，下方放不下 → 翻到触发钮上方：top = 700 - 101 - 4
+    expect(menu!.style.top).toBe('595px')
+    expect(menu!.style.left).toBe('100px')
+    expect(menu!.style.width).toBe('200px')
     await act(async () => { root.unmount() })
     container.remove()
   })