[Bug]: A new preview tab opens on another machine's window instead of the one showing the thread

[mkiera]

Before submitting

• I searched existing issues and did not find a duplicate. preview_evaluate and preview_snapshot silently read a different browsing context when two automation hosts are registered #13051 / fix(preview): use the visible browser for new agent sessions #13064 made new sessions prefer the client that owns or shows the tab. This report is the case where no tab exists yet. [Bug]: After a preview automation timeout, the session fails over to a client on another machine that doesn't have the thread open #13540 covers a session moving after a timeout, which is a different path.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Run the T3 Code desktop app on machine A. The server runs there too.
2. Open the desktop app on machine B and connect it to machine A's environment. Open any other thread there. Leave its browser panel closed.
3. On machine A, open a thread that has no browser tab yet.
4. Click into machine B's T3 window so it is the most recently focused window. Then return to machine A.
5. Ask the agent in the machine A thread to open a browser tab (preview_open with reuseExistingTab: false).

Expected behavior

The tab opens in machine A's window, which is showing the thread.

Actual behavior

The tab opens on machine B, in a thread nobody is looking at. It uses machine B's network and browser profile, so local URLs such as http://127.0.0.1:<port> fail and the user's sign-ins from machine A are missing. The session then stays pinned to machine B until that client disconnects.

When no host owns a tab for the thread, the broker ranks hosts by focused and focusOrder only. Hosts do not report which thread they are showing, so the broker cannot prefer the window the user is watching.

Impact

Major degradation or frequent failure

Version or commit

T3 Code (Nightly) 0.0.43-nightly.20260925.2251

Environment

Machine A: Windows 11 x64, desktop app and server. Machine B: Windows 11 x64 VM, desktop app connected to machine A through T3 Connect. Agent provider: Claude Code.

Workaround

Click into the window showing the thread before the agent opens a new browser tab.

[juliusmarminge]

Open PR #13773 already targets this with Fixes #13771 (broker prefers hosts reporting the thread via activeThreadId when no tab exists yet). Leaving this open for that land.

Related but different path: #13540 (failover after automation timeout). Prior art for “prefer the visible client when a tab already exists”: #13051 / #13064.