[BUG] Hidden Browser pane: screenshots time out 30s; preview_start no longer auto-opens the pane (regression; follow-up to #76649, NOT a dupe of #77670)

Summary

In Claude Desktop on Windows, every Browser pane computer { action: "screenshot" } (and zoom) call times out after 30s whenever the Browser pane is not visible in the app window. All non-capture browser tools (read_page, get_page_text, read_console_messages, clicks, form_input) keep working instantly against the same tab, so the page/renderer is healthy — only the capture path hangs.

Two linked defects, and the second is why this bites so hard:

1. Hidden pane ⇒ captures never resolve. Chromium doesn't composite frames for a hidden webview, so CDP Page.captureScreenshot never returns. On teardown the pending commands flush as:

``
[CDPTools] CDP compressed screenshot failed: target closed while handling command
``

1. The agent can no longer open the preview pane itself (regression). When I first installed the app, the agent starting a preview (preview_start) would open/reveal the pane on its own, so captures Just Worked. In my current environment it no longer does — preview_start succeeds, the tab exists and is fully scriptable, but no pane appears, and every screenshot then times out until I manually open the pane.

If I manually open the Browser pane, the very next screenshot returns instantly, every time. Confirmed in both directions repeatedly between 2026-07-14 and 2026-07-16.

Why this is not a duplicate of #77670

In #77670 the reporter describes the preview panel as open and screenshots failing regardless of what site is loaded. Here it's the opposite: pane visible ⇒ captures work 100% of the time; pane hidden ⇒ captures fail 100% of the time. The failure is strictly a function of pane visibility, not of the loaded site. (I originally reported this on #76649, which was closed on 2026-07-16 — the behavior still reproduces on the build installed after that closure, hence this fresh issue. Last comment there with this detail: https://github.com/anthropics/claude-code/issues/76649#issuecomment-4994890532)

Environment

• Windows 11 Pro 10.0.26200, NVIDIA GPU
• Claude Desktop (MSIX): build 1.21459.0.0 as of 2026-07-14; still reproduces after the subsequent app update installed 2026-07-16
• Embedded Claude Code (CCD): 2.1.209
• Local Vite dev server (http://localhost:5173), but the site is irrelevant — see above

Steps to reproduce

1. Launch Claude Desktop; do not open/reveal the Browser pane.
2. Start a Claude Code session and have the agent run preview_start (dev server or plain URL). Observe the pane does not auto-open.
3. Agent calls computer { action: "screenshot" } → times out after 30s with "The Browser pane may be stuck (modal dialog, navigation hang, or unresponsive renderer)."
4. Same turn: get_page_text / read_page / read_console_messages on the same tabId return instantly with correct content and a clean console.
5. Manually make the Browser pane visible; retry the screenshot → returns immediately.

What we've tried (all ruled out)

• Full app cache wipe (2026-07-14) — no change.
• App update + NVIDIA driver update (2026-07-14) — no change.
• Retest after the update that followed #76649's closure (2026-07-16) — still reproduces exactly as before.
• Resetting persisted pane layout (2026-07-16): claude_desktop_config.json → preferences.epitaxyPrefs."desktop-frame.paneStore.v1" had extraPanesByMode: {} (no panes in any mode), theory being that a persisted pane-less layout prevented auto-open. Quit the app fully, cleared the entry, relaunched — refuted: pane still doesn't auto-open on preview_start, hidden-pane screenshots still time out at 30s while text-based tools respond instantly.

Expected

Either preview_start reveals the Browser pane (as it did on first install), or capture works for a hidden pane (e.g. force compositing / use an offscreen capture path), or the tool fails fast with an actionable message ("Browser pane is hidden — captures require a visible pane") instead of a 30s timeout blaming the renderer.

Workaround

Manually open the Browser pane before any screenshot; captures then succeed immediately and reliably.

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🤖 Filed with Claude Code on the user's behalf; repro details gathered in-session.