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).

src/client/sidebar.module.css

@@ -48,7 +48,18 @@
      hidden box stays a PROGRAMMATICALLY scrollable container, and a
      scrollIntoView fired from panel content (the file tree's "Show in
      folder" reveal) scrolled the host itself — shifting the whole panel,
-     tab bar included, out of the viewport with no way to scroll back. */
+     tab bar included, out of the viewport with no way to scroll back.
+     The same class covers any scroll-into-view fixup — focus moving into
+     an off-viewport region, a nested iframe/workbench claiming focus
+     while it loads — so the guard belongs on the layer itself, not per
+     call site. The `hidden` declaration BEFORE `clip` is an old-engine
+     fallback, not dead style: engines without `clip` support (Safari < 16)
+     drop the `clip` line, which would leave this box `overflow: visible`
+     — no clipping at all, and the page-draggable regression above
+     returns. With `hidden` first those engines keep the clipping (plus
+     its pre-clip scrollability) while every clip-aware engine applies
+     the later `clip`. */
+  overflow: hidden;
   overflow: clip;
 }