Fix thread detachment when re-opening live session via new sidebar

[lukemarsden]

Summary

After the two upstream-merge PRs (#42 / #43) landed the new agents sidebar, clicking the currently-open thread in the sidebar caused the panel to "detach" from the live Entity<AcpThread>: Zed's panel stopped updating but Helix kept receiving message_added / message_completed events for the same session. Single-thread, single-click was enough to trigger it — no ThreadSwitcher, no restart.

Root cause: two parallel dedup paths in agent_panel.rs use different identity checks. notify_thread_display (Helix bring-up) compares Entity<AcpThread> references; panel.load_agent_thread (UI clicks) compares acp::SessionId. When the session-id check missed, external_thread issued a duplicate connection.load_session(), producing a fresh entity Y. Helix's WebSocket subscription stayed on the original X (events kept flowing) but the panel rebound to Y (silent) — the same split-brain shape as d7be64fad1 (restart variant), different trigger.

Fix

Adds an #[cfg(feature = "external_websocket_sync")]-gated guard at the top of pub fn load_agent_thread in crates/agent_ui/src/agent_panel.rs, before the existing has_session block:

1. Look up external_websocket_sync::get_thread(session_id).
2. If a live entity exists → entity-id check against the active CV → no-op.
3. Else entity-id check against retained CVs → promote the match.
4. Else wrap the live entity via ConversationView::from_existing_thread(…) (same path notify_thread_display uses).
5. Else fall through to the unchanged upstream logic.

The new code is purely additive in front of the existing function body. Outside the Helix feature, behaviour is unchanged.

Changes

• crates/agent_ui/src/agent_panel.rs — entity-identity guard at the top of load_agent_thread.
• portingguide.md — Critical Fix Upstream merge 2026 02 26 #11 documenting the guard, its rebase requirements, and its relationship to the previous unregister_thread_if_matches fix.

Notes

• No live repro / E2E in this environment. Host has no Rust toolchain and no Docker access for the run_docker_e2e.sh setup. Build was attempted via ./stack build-zed dev (zed-builder docker image); status noted in the spec task. Reviewer to run the manual repro and the E2E script before merging.
• Hypothesis-agnostic. The design considered three possible reasons the session-id dedup could miss (root_session_id None/stale, session-id mismatch between paths, BaseView::Uninitialized at click time). The entity-id guard short-circuits all three without needing to confirm which is hit. If a follow-up investigation finds a specific root cause, a smaller upstream fix is also possible — but this guard is the safe Helix-side floor.

Release Notes:

• Fixed thread becoming detached when clicking the currently-open thread in the new agents sidebar (Helix mode).

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