Embedded preview pane: webview overlays entire window (covers chat/sidebar) + preview_screenshot times out - Windows 11, desktop 1.9659.2

## Summary
On the Claude desktop app (Code / preview pane), the embedded localhost preview is unusable due to three compounding issues:

1. Webview renders as a full-window overlay. The preview content paints across the entire window — covering the chat input, the session sidebar, and the code/log panes — instead of staying docked in the right-hand preview pane. The pane's address bar ("localhost:3000") is correctly positioned at the top-right, but the webview content is decoupled from those bounds and covers everything.
2. preview_screenshot times out after 30s ("renderer may be stuck / unresponsive"), even though the page itself loads fine (evidence below). This also breaks the agent / visual-review screenshot pipeline.
3. Chat becomes unusable while the preview is open. Clicking into the chat input breaks the layout and steals focus — typing is impossible; input "doesn't land in the right place."

This looks related to the (now-closed) #57253, but it persists on a much newer build, and the dominant symptom is now webview bounds/positioning + the screenshot pipeline — not localhost being blocked.

Environment

• Claude desktop app: 1.9659.2 (MSIX package Claude_pzs8sxrjxfjjc)
• Claude Code engine: 2.1.156
• OS: Windows 11 Home, build 26200 (10.0.26200), x64
• Previewed site: a Next.js 15.5 (Turbopack) app on http://localhost:3000 — RTL, fully embeddable (no X-Frame-Options / CSP / frame-busting; loads perfectly in a normal browser)

Evidence the previewed app is healthy (i.e. this is desktop-app-side)

• Dev-server log: Compiled / in 9.3s, then repeated GET / 200 in <1s.
• Console from inside the preview webview: React mounts, a [Vercel Web Analytics] [view] http://localhost:3000/ pageview fires, and [Fast Refresh] done. So the page does load and run inside the webview — yet it paints in the wrong bounds and preview_screenshot still times out.

Steps to reproduce

1. Open a project, start a dev server on port 3000, open the preview pane pointed at localhost:3000.
2. The page loads, but the webview covers the whole window (overlapping chat + sidebar).
3. Click the chat input -> layout breaks, cannot type.
4. Run preview_screenshot -> times out after 30s.

Already tried (none resolved it)

• Verified on a current/fixed build (1.9659.2, well past #57253).
• Full OS reboot.
• Cleared GPU/render caches (GPUCache, DawnGraphiteCache, DawnWebGPUCache, Code Cache, Cache) and the Partitions\launch-preview-* webview partitions.
• Maximize / restore / drag-resize the window (to force webview bounds recalculation).
• Toggle the preview pane closed and reopen.
• Reset zoom (Ctrl+0).

Expected

Webview stays confined to the preview pane; chat stays interactive; preview_screenshot returns an image.

Actual

Webview overlays the entire window; chat is unusable while preview is open; screenshot times out.

I can provide a screen recording and full logs on request. Please keep me posted on a fix or reliable workaround — the embedded pane matters specifically because it provides screenshot capture + element selection for the agent, which an external browser does not.