[BUG] Desktop Browser pane: popup windows are not supported, so OAuth/payment/popup-only sites cannot be used

Problem

The desktop app's built-in Browser pane does not support popup windows. Some websites only work through a popup: OAuth / "Sign in with ..." flows, payment and 3-D Secure windows, consent dialogs, document previews, and some admin consoles that open a task in a new window.

In the pane, the popup is either blocked or never appears, and there is no "allow popups for this site" option. On those sites the flow cannot be completed at all, so the site is effectively unusable in the pane.

What I'd expect

• Popups (window.open, target=_blank with an opener) open as a new tab or a child window inside the Browser pane, with window.opener / postMessage working so OAuth and payment flows can return to the parent page.
• A per-site "allow popups" toggle if they are blocked by default, plus a visible notice when one was blocked.
• The agent can see and act on the popup tab (it shows up in the tab list).

Related

• #76582 (closed: window.open returns null from cross-site iframes). That fixed the iframe case; top-level popups still do not work for the sites above.
• #96516 (popup sign-ins unreachable in Claude in Chrome)

Environment

• Claude desktop app 2.31226.1, Claude Code 2.1.296
• macOS 27.0.1