fix(desktop): stop panel host from blocking macOS window drag

The shell's base stylesheet makes every direct body child no-drag
(`html[data-platform=darwin] body > :not(#root)`). The viewport-sized
`[data-dsh-panel-host]` layer inherited it, and since app-region ignores
pointer-events it cancelled every window-drag strip beneath it once the
shell recomputed regions.

Take the host and layer out of the app-region computation (`!important`,
the shell selector carries an id's specificity) and keep the panels
themselves no-drag so their controls still receive clicks.

Fixes #772

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/client/sidebar.module.css

@@ -63,6 +63,22 @@
   overflow: clip;
 }
 
+/* Desktop drag regions ignore pointer-events: the shell's base stylesheet
+   makes every direct body child `no-drag`, and this viewport-sized layer
+   inherited it, cancelling every window-drag strip beneath it once the
+   shell recomputed regions (window drags worked once, then stopped).
+   Take the host and layer out of the app-region computation; `!important`
+   because the shell's `body > :not(#root)` selector outranks ours. The
+   panels themselves stay no-drag so their controls keep receiving clicks. */
+:global(html[data-platform] body > [data-dsh-better-sidebar]),
+:global([data-dsh-panel-host]) {
+  -webkit-app-region: initial !important;
+}
+
+:global([data-dsh-panel-host]) > * {
+  -webkit-app-region: no-drag;
+}
+
 :global([data-dsh-panel-host][data-dsh-panel-host-degraded]) {
   position: absolute;
   top: 0;

tests/e2e/mount.e2e.ts

@@ -615,6 +615,16 @@ 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).
+  const hasHostReset = await page.evaluate(() => {
+    for (const tag of document.querySelectorAll('style')) {
+      if (tag.textContent !== null && /\[data-dsh-panel-host\]\{-webkit-app-region:\s*initial\s*!important/.test(tag.textContent)) return true
+    }
+    return false
+  })
+  expect(hasHostReset, 'the panel host must opt out of app-region computation').toBe(true)
 })
 
 test('standard WCO geometry drives the strip reactively (issue #257)', async ({ page }) => {