[Bug][Desktop v2] Middle-clicking a subagent opens a separate Not Found window

Description

In the Desktop v2 interface, middle-clicking a subagent/task card does not open the child session as a new OpenCode tab while keeping the parent tab. Instead, it opens a separate OpenCode-like window/page and displays Not Found.

A normal click can navigate to the child session in the current app. The failure is specific to the middle-click/new-tab path.

Plugins

oh-my-openagent (v4.19.1, personally forked)

OpenCode version

1.18.4

Steps to reproduce

1. Open an existing session that contains a completed or running subagent/task card.
2. Ensure the Desktop v2/new layout is enabled.
3. Middle-click the subagent/task card.
4. Observe that a separate OpenCode-like window/page is opened instead of a new tab in the existing window.
5. Observe Not Found in the new window/page.

Expected behavior

The parent session remains available in its current tab, and the child session opens in a new OpenCode tab in the same Desktop window, matching the behavior of middle-clicking a session from the Home session list.

Actual behavior

The native link behavior escapes the in-app tab system and opens a separate window/page. The new page cannot render the child route and shows Not Found.

Screenshot and/or share link

No response

Operating System

Windows 11

Terminal

OpenCode Desktop (not applicable)

Additional context

This appears to be a stock OpenCode GUI issue rather than an oh-my-openagent issue:

• The task card is rendered as a native <a> by packages/session-ui/src/components/basic-tool.tsx.
• packages/session-ui/src/components/message-part.tsx only intercepts button === 0; middle-click returns before preventDefault() and before the app tab store is used.
• The Home session list already has an onAuxClick path that calls tabs.addSessionTab().
• The child URL shape and /server/:serverKey/session/:id route are valid for the normal in-app navigation path.
• The Desktop Electron shell does not configure a setWindowOpenHandler for this native middle-click path.

Potential fix: handle middle-click/auxclick on the task card and add the child session through the existing app tab store, then add a Desktop v2 regression test covering parent-tab preservation and child-tab opening.

Related reports/changes: #34808, #35577, #15077, and the middle-click feature added in v1.17.19.