fix(shell): keep an overflow:hidden fallback beneath clip + guard the cascade

shift fix) — but as a BARE declaration. Engines without clip support
(Safari < 16, older Chromium) drop an unsupported declaration at parse
time, which leaves the host `overflow: visible`: no clipping at all,
and the historical "collapsed panels extend the document's scrollable
area, whole page draggable" regression returns on those engines.

Declare `overflow: hidden` BEFORE `overflow: clip`: clip-aware engines
apply the later clip (identical behavior to #453), while old engines
fall back to hidden and keep clipping (with its pre-clip
scrollability — the same state they had before #453, strictly better
than visible). The cascade order is the contract.

Also extends the rule comment (the fix guards the whole
scroll-into-view class — focus moving into off-viewport regions,
nested iframes/workbenches claiming focus while loading — not just the
reveal case) and adds tests/panel-host-css.spec.ts guarding the
cascade (hidden first, clip last), the fixed viewport layer, and that
the degraded rule never reintroduces overflow. Contract documented in
the integration guide §12 (root anchor bullet).

docs/external-plugin-guide.md

@@ -752,7 +752,7 @@ ctx.effect(() =>
 
 - **面板表面**：右/底面板背景 = `var(--dsw-alias-bg-layer-1)`。**绝不消费 `--dsw-specific-sidebar-fill`**（宿主左导航专属，皮肤按左导航语义覆盖它，面板消费会失去填充）。换面板表面 = 覆写 `--dsw-alias-bg-layer-1`。
 - **终端/编辑器表面**：`effectiveTokenValue` 读 `--dsw-alias-bg-base`——`transparent` 与 alpha < 0.9 的半透明值回退不透明底色（文字不叠背景画，issue #90）；≥ 0.9 放行。
-- **根锚点**：宿主 div 带 `data-dsh-better-sidebar`（append 到 body）；其内**面板宿主层** `[data-dsh-panel-host]`（`fixed; inset:0; z-25; pointer-events:none`，v0.13.1+），面板/开关簇 absolute 定位，免疫中间层 transform 劫持；页面级 transform 触发 `data-dsh-panel-host-degraded` 降级。皮肤作用域覆盖限定在 `[data-dsh-better-sidebar]` 内。
+- **根锚点**：宿主 div 带 `data-dsh-better-sidebar`（append 到 body）；其内**面板宿主层** `[data-dsh-panel-host]`（`fixed; inset:0; z-25; pointer-events:none; overflow:hidden+clip`，v0.13.1+），面板/开关簇 absolute 定位，免疫中间层 transform 劫持；页面级 transform 触发 `data-dsh-panel-host-degraded` 降级。`overflow` 级联是**契约**（`hidden` 兜底 + `clip` 收尾，`tests/panel-host-css.spec.ts` 守护）：`hidden` 盒子仍是滚动容器，脚本滚动或浏览器 scroll-into-view 修正（焦点移入视口外区域、嵌套 iframe/工作台加载时抢焦点、面板滑出动画中 focus() 落点）会沿最近可滚祖先滚走整层——面板与开关簇集体偏离视口角（computed left/right 仍"正确"，偏移藏在盒子自身 scroll offset 里）；`clip` 裁剪语义相同但不产生滚动盒，任何路径都滚不动这层。皮肤作用域覆盖限定在 `[data-dsh-better-sidebar]` 内。
 - **布局变量**（`<html>` 上，面板打开时有效）：`--dsh-sidebar-width` / `--dsh-sidebar-height`。右面板宽度 = AppFrame 的 `padding-right` 预留（新版 `#root [data-dsh-frame]` / rc.8 `#root > [data-slot="root"] > div` 双锚点），AppFrame border box 保持完整桌面视口宽度（Harness 以此判定桌面/窄屏布局，避免插件面板展开误入窄屏）；AppFrame 的 details 拖拽手柄按同一变量向左平移贴合列边缘。底部面板仍走 centerCol `margin-bottom`；centerCol 锚点 = **JS 标注**（禁止 `nth-child`）：侧栏 shell 的定位器给测得的 centerCol 节点打 `[data-dsh-center-col]` 标签（`Sidebar.tsx` locate，节点更换/HMR 时随 ref 迁移），`layout.css` 用 `#root [data-dsh-center-col]` 选中（`drag-layout.e2e.ts` 断言恰一节点且为 `[data-slot="conversation"]` 的父级；frame 宽度与桌面 Session Log 由 `desktop-layout.e2e.ts` 断言）。
 - **桌面信号与标题栏**（v0.14.1+ 四方案模型 `SidebarPrefs.titleBarScheme`，唯一决策点 `src/client/titlebar-strip.ts` 纯函数）：
   - 壳信号（只读，不自动触发修改）：URL `dsh-desktop-mode` / `dsh-desktop-platform` / 可选 `dsh-desktop-titlebar-inset`（0–120 clamp）。