feat: session detach & reconnect — tabs detach, only exit kills

[datasciencemonkey]

Summary

Sessions now survive tab closure. Closing a tab detaches the frontend — the PTY process keeps running. Only typing exit in the shell kills a session. Users can reconnect to existing sessions on return visits via session picker.

What Changed

Area	Before	After
Tab/pane close	sendBeacon kills PTY immediately	Frontend detaches, PTY stays alive
Process exit (exit)	Marks exited=True, lingers in dict	Calls terminate_session() — immediate cleanup
Return visit (PAT valid)	Always creates new session	Checks for existing sessions first
1 existing session	N/A	Auto-reattaches with buffer replay
N existing sessions	N/A	Interactive picker (keyboard)
Session list API	N/A	GET /api/sessions with process detection + label
Session attach API	N/A	POST /api/session/attach with buffer replay
Split pane / new tab	N/A	Always creates fresh session (no picker)
Reattach display	N/A	Skips CoDA splash, replays buffer, sends resize
Session management	N/A	☰ toolbar button, Ctrl+Shift+S hotkey, d{N} to kill
Session naming	N/A	Tab label stored on create, shown in picker
Duplicate guard	N/A	Already-open sessions show (open), can't attach twice

Session Picker

  Existing sessions:

  1  Shell 1        (7m ago)        [claude] (open)
  2  Shell 2        (7m ago)        [node]
  3  My Agent       (6m ago, idle 5m) [bash]

  n  New session
  dN Kill session N (e.g. d2)
  x  Kill all and start fresh

  Select: _

Access via: ☰ toolbar button, Ctrl+Shift+S, or on page load with existing sessions.

Test Plan

Automated (122 tests, all passing)

uv run pytest tests/ -v --ignore=tests/test_npm_version_pinning.py

Manual Testing on Databricks Apps (test-coda on 9cefok)

1. Detach — tab close no longer kills session

• Open app, create terminal sessions
• Close the browser tab
• Reopen the app — sessions still alive (confirmed across all deployments 03:46-04:34)
• No POST /api/session/close in logs after tab close (only WS disconnect)

2. Auto-reattach (1 session)

• With 1 existing session, auto-reattaches via POST /api/session/attach
• Buffer replayed (1.5KB-128KB payloads confirmed)
• Joins WS room after attach
• See "── reattached to ──" message visually
• Terminal redraws correctly without manual resize

3. Session picker (multiple sessions)

• With 2+ sessions, picker shown (GET /api/sessions at 04:11:48, 04:33:21, 04:34:27)
• User selected session from picker → attached (04:11:53, 04:12:02)
• ☰ / Ctrl+Shift+S opens picker mid-session (04:11:58, 04:33:21, 04:34:27 — GET /api/sessions + leave WS room)
• d{N} kills individual session from picker (Terminating 22ff502b at 04:33:36 + POST /api/session/close at 04:33:39)
• Killed session not in subsequent picker (04:34:27 — 578 bytes, one fewer session)
• Type n → creates new session
• Type x → kills all, creates fresh

4. Fresh deploy — PAT flow then new session

• After restart, no existing sessions
• PAT prompt → configured → setup → new session created (confirmed 04:31-04:32)

5. Split pane / new tab — always fresh

• Split pane creates fresh session (no picker)
• New tab creates fresh session — POST /api/session at 04:33:04, 04:33:19, 04:34:17 (no /api/sessions call)

6. Exit — kills session immediately

• exit → Session process exited → Terminating → Force killed (04:12:32)
• Session removed from dict — not in next GET /api/sessions response

7. Process detection

• GET /api/sessions returns process names (confirmed in all responses)
• Verify claude shows correctly in picker

8. Buffer replay on reattach

• Attach returns buffer (confirmed multiple payloads 1.5KB-128KB)
• Terminal shows prior output after reattach visually

9. Reattach skips CoDA splash

• Reattach shows replayed buffer instead of splash screen

10. Session labels

• Picker shows tab name instead of just process name
• Process name shown in brackets as secondary info

11. Already-open guard

• Open sessions show (open) tag in picker
• Typing number for open session shows "Already open" message

12. PAT rotation continues

• Rotation with ELIMINATED confirmation (04:02:19, 04:13:55)

This pull request was AI-assisted by Claude Code.