Nth-worker tab spawns sometimes never initiate: lazy tab init + focus/restore race; new_split path never focuses the new tab

[EtanHey]

Etan hit this live 2026-08-09 (a Codex-spawned worker never initiated until he happened to click its pane). Cursor recon (docs.local/research/focus-flakiness-recon.txt, file:line cited) explains the intermittence:

Mechanism: cmux lazy-inits TABS. The Nth worker in a workspace docks as a tab into the right-column pane (layout-policy.ts:410-420) while global focus stays with the lead. #353 added a brief surface.focus before launch (agent-engine.ts:4819-4829), but (a) the focus→origin-restore window (server.ts:5463-5493) can race too short, and (b) non-spawn_agent paths (new_split + boot_prompt_path, default focus:false) never call focusSurface on the new tab at all — only workspace-select + restore.

Why it usually works: first worker is a SPLIT (often selected-in-pane), and #353's focus usually holds long enough.

Ask: make pane init deterministic — hold focus until launch evidence (CLI banner) exists, or focus-pump lazy tabs until init, and give the new_split path the same #353 treatment. Confirming evidence checklist is in the recon file (§3).

Refs: stability plan cluster B (focus-robustness).

— cmuxlayerClaude (lead) · claude-code/claude-fable-5

[EtanHey]

Pre-registered reproduction attempt (stability plan B2, 2026-08-10 night): scripted spawn of N=1..4 tabs into an unfocused right pane, 3 full runs, asserting CLI start within 15s. NOT REPRODUCED — 0/3 failures at every N; split and focus:true controls initiated 3/3. Script + result: cmuxlayer docs.local/research/b2-reproducer.sh / b2-repro-result.md.

The race is real (it bit live 2026-08-09) but is NOT deterministic under these conditions — the fix hypothesis in this issue should not assume an easy repro; instrumentation on the focus→restore window may be needed to catch the failing timing. Prediction record: the reproducer hypothesis was falsified; honest negative per the pre-registration law.

— cmuxlayerClaude (lead) · claude-code/claude-fable-5