fix(review): 按独立 review（Chromium 实测）修正 #786 的措辞、守卫与覆盖

reviewer 在本机跑 Chromium 逐条实测（同一批宿主规则 + 插件产物），结论：修复方向与形状正确
（`.mermaidModal` 预修复计算 `no-drag`、修复后 `none`；级联探针确实能把未修复产物判红），
但有几处措辞与守卫不到位。逐条处理：

1. **「app-region 会被继承」的说法在该引擎里不成立**：`[data-dsh-panel-host]` 不是 body
   子元素（DOM 是 `body > [data-dsh-better-sidebar] > [data-dsh-panel-host]`），lane 用的
   Chromium 不传播该属性（实测：`drag` 父元素里子元素读 `none`）。改写成「宿主是 body
   子元素；在 Electron 里该值会传到它内部的面板层（#772 的 CDP 实测），lane 的引擎不传播，
   所以面板层自己也声明同一条规则」——CSS 注释、指南 §12.1、探针注释三处同步。
2. **`regions.layer` 那条断言是真空断言**（修复前后都读 `none`）：保留为回归钉子，但在
   注释里明确标注它不具判别力、面板层的规则由 `panel-host-css.spec.ts` 钉住。
3. **给修复本身补行为级守护**：放大视图已经在同一 lane 里被真实打开过，于是在那里直接对
   `[data-mermaid-modal]`（以及它的按钮）读计算值——断言用**页面里宿主自己的** darwin
   规则（不再是探针复制的副本）。实测判别力：npm 0.22.0 上 `Received: "no-drag"`、修复版
   `none`。
4. **「交互层不要加 initial」与放大视图的矛盾**：放大视图是**遮罩层例外**——模态语义下
   只有其中控件必须保持 `no-drag`；残余代价（背景落在顶部拖拽带上时点背景会变拖窗）已写进
   指南，避免后来者按字面判据「修」掉其中一边。
5. **`.selectionPopup` 的负向守卫可被选择器列表绕过**（reviewer 实测：`.selectionPopup,.x
   { initial !important }` 能通过）：改成按**声明块**扫描——凡带 `-webkit-app-region:
   initial` 的块，其选择器文本都不得出现 `selectionPopup`。
6. **补上 `none` 关键字的坑**：字面量 `none` 计算成 `no-drag`（是扣减值），只有 `initial`
   /`unset`/`revert` 才是中性值——CSS 注释、指南与单元用例注释都已写明。
7. **未覆盖层给出结论**：`FileTree` 的 `.uploadDropChatHint` 是 body 直挂的装饰层，但只在
   OS 文件拖拽悬停期间存在且 `pointer-events:none`，收益极小而**有意不处理**（写进指南，
   不再让「覆盖清单」看起来是绝对集合）。
8. 顺带：`§12.2` 的同步集里 `tests/theme.spec.ts` 只守颜色（本轮无颜色改动），形状守护落在
   已解析该样式表的 `tests/panel-host-css.spec.ts` 上——指南那句已按此表述。

验证：typecheck / lint 通过；panel-host-css 4 例通过；本地挂载 lane 7 passed（含新的放大
视图计算值断言）；对 npm 0.22.0 的判别力实测：宿主探针红（`host` = `no-drag`）、放大视图
断言红（`modal` = `no-drag`），修复版全绿。

tests/e2e/mount.e2e.ts

@@ -517,6 +517,28 @@ test('plugin mounts into the DSH shell and survives a built-in tab sweep', async
   const modal = page.locator('[data-mermaid-modal]')
   await pane.locator('[data-mermaid-diagram] svg').first().click()
   await expect(modal, 'clicking the diagram must open the zoom modal').toHaveCount(1, { timeout: 10_000 })
+  // While it is up, the modal is the plugin's OTHER viewport-sized BODY CHILD:
+  // without the app-region reset the shell's blanket `body > :not(#root)`
+  // no-drag makes it cancel every window-drag strip (and the macOS
+  // double-click-title zoom) until it closes — issue #772. Measured against the
+  // SHELL's own darwin rules (present in this page's stylesheet, activated by
+  // the platform attribute) rather than a copy: the modal must compute the
+  // neutral `none`, while its toolbar buttons keep `no-drag` so a click on them
+  // is never a window drag.
+  const modalRegions = await page.evaluate(() => {
+    const previous = document.documentElement.dataset.platform
+    document.documentElement.dataset.platform = 'darwin'
+    const modalElement = document.querySelector('[data-mermaid-modal]')
+    const region = (element: Element | null): string => element === null
+      ? 'missing'
+      : getComputedStyle(element).getPropertyValue('-webkit-app-region').trim()
+    const result = { modal: region(modalElement), button: region(modalElement?.querySelector('button') ?? null) }
+    if (previous === undefined) delete document.documentElement.dataset.platform
+    else document.documentElement.dataset.platform = previous
+    return result
+  })
+  expect(modalRegions.modal, 'the zoom modal must opt out of app-region computation').toBe('none')
+  expect(modalRegions.button, 'its buttons must stay no-drag').toBe('no-drag')
   await page.keyboard.press('Escape')
   await expect(modal, 'Esc must close the zoom modal').toHaveCount(0, { timeout: 10_000 })
   await assertNoCrash()
@@ -615,15 +637,17 @@ test('conservative auto: URL stamps alone never modify the layout; plugin chrome
     return false
   })
   expect(hasNoDragRule, 'the bundle must ship the drag-region opt-out rule').toBe(true)
-  // The viewport-sized panel host must NOT inherit the shell's blanket
-  // `body > :not(#root) { no-drag }` — it would cancel every window-drag
-  // strip beneath it on macOS (issue #772). A text match only proves the
-  // declaration shipped, so this probe reproduces the SHELL's two darwin
-  // rules verbatim, flips the platform attribute the shell stamps, and reads
-  // the COMPUTED property: app-region is inherited and ignores
-  // pointer-events, so the loss shows on the host/layer — never on the strip
-  // itself (it computes `drag` in every configuration, which is why a fix
-  // validated only against the strip proves nothing).
+  // The plugin host is a direct body child, so on macOS it picks up the
+  // shell's blanket `body > :not(#root) { no-drag }` and — because app-region
+  // ignores pointer-events — the viewport-sized layer inside it cancelled every
+  // window-drag strip beneath it (issue #772). A text match only proves the
+  // declaration shipped, so this probe reproduces the SHELL's two darwin rules
+  // verbatim, flips the platform attribute the shell stamps, and reads the
+  // COMPUTED property. Two things to know when reading the expectations below:
+  // the strip itself computes `drag` in every configuration (a fix validated
+  // against the strip alone proves nothing), and `[data-dsh-panel-host]` is NOT
+  // a body child (it lives inside the host), so its own value is pinned by the
+  // unit guard rather than by this probe — see the `layer` expectation.
   const regions = await page.evaluate(() => {
     const style = document.createElement('style')
     style.textContent = [
@@ -660,8 +684,13 @@ test('conservative auto: URL stamps alone never modify the layout; plugin chrome
     return result
   })
   expect(regions.strip, 'the drag strip itself must keep computing `drag`').toBe('drag')
-  expect(regions.host, 'the plugin host must not inherit the shell no-drag').toBe('none')
-  expect(regions.layer, 'the panel host must not inherit the shell no-drag').toBe('none')
+  expect(regions.host, 'the plugin host must not take the shell blanket no-drag').toBe('none')
+  // NOT discriminating here, and kept as a regression nail only: the layer is
+  // not a body child, and this engine does not propagate the property into it,
+  // so it reads `none` pre- and post-fix. Its rule is pinned by
+  // tests/panel-host-css.spec.ts; in Electron (#772's CDP reading) the layer
+  // DOES end up no-drag through that propagation.
+  expect(regions.layer, 'the panel layer must keep its own reset').toBe('none')
   expect(regions.layerChild, 'panels stay no-drag so their controls keep receiving clicks').toBe('no-drag')
 })
 

tests/panel-host-css.spec.ts

@@ -49,12 +49,14 @@ describe('panel host layer css', () => {
     // The shell's `html[data-platform=darwin] body > :not(#root) { no-drag }`
     // carries an id in its specificity, so only `initial !important` wins;
     // `initial` computes to the NEUTRAL `none` (does not subtract from the
-    // drag region) while `no-drag` is the subtracting value — hence the
-    // split: decorative viewport-sized layers opt out, their panels stay
-    // no-drag so controls keep receiving clicks. Dropping the `!important`
-    // silently re-breaks window drag / double-click-title zoom on macOS
-    // (there is no macOS runner to catch it), so pin the shape here where
-    // Linux `pnpm test` fails instead.
+    // drag region) while `no-drag` is the subtracting value — and the literal
+    // keyword `none` is NOT neutral (it computes to `no-drag`), which is
+    // exactly why this guard pins `initial`. Hence the split: decorative
+    // viewport-sized layers opt out, their panels stay no-drag so controls
+    // keep receiving clicks. Dropping the `!important` silently re-breaks
+    // window drag / double-click-title zoom on macOS (there is no macOS
+    // runner to catch it), so pin the shape here where Linux `pnpm test`
+    // fails instead.
     const text = stripComments(css)
     expect(text, 'the panel host must opt out of app-region computation').toMatch(
       /:global\(\[data-dsh-panel-host\]\)\s*\{[^}]*-webkit-app-region:\s*initial\s*!important/,
@@ -62,12 +64,21 @@ describe('panel host layer css', () => {
     expect(text, 'panels must stay no-drag').toMatch(
       /:global\(\[data-dsh-panel-host\]\)\s*>\s*\*\s*\{[^}]*-webkit-app-region:\s*no-drag/,
     )
-    expect(text, 'the mermaid zoom modal is the other viewport-sized body child').toMatch(
+    expect(text, 'the mermaid zoom modal is the other persistent viewport-sized body child').toMatch(
       /\.mermaidModal\s*\{[^}]*-webkit-app-region:\s*initial\s*!important/,
     )
     // The interactive popups must NOT get `initial`: they are buttons/handles
     // the shell already keeps no-drag, and outranking that rule makes a press
-    // start a window drag (the #103/#111 click-swallow class).
-    expect(text).not.toMatch(/\.selectionPopup\s*\{[^}]*-webkit-app-region:\s*initial/)
+    // start a window drag (the #103/#111 click-swallow class). Checked per
+    // DECLARATION BLOCK, not per single-selector rule: a selector list
+    // (`.selectionPopup, .other { … initial !important }`) would slip past a
+    // simple "class immediately followed by {" pattern.
+    const resets = [...text.matchAll(/([^{}]+)\{([^{}]*)\}/g)]
+      .filter(([, , body]) => /-webkit-app-region:\s*initial/.test(body ?? ''))
+    expect(resets.length, 'the file must declare the app-region opt-out').toBeGreaterThan(0)
+    const resetSelectors = resets.map(([, selector]) => selector ?? '').join(',')
+    for (const excluded of ['selectionPopup']) {
+      expect(resetSelectors, `${excluded} must not opt out of app-region`).not.toContain(excluded)
+    }
   })
 })