feat: tangent - keep track of open tabs while switching sessions inside project

[maxy-shpfy]

Description

Workarea tabs and session tabs are now scoped per-session rather than being reset on every session switch. When the active session changes, the previously open tabs for that session are restored instead of cleared. Backgrounded sessions' tabs unmount naturally, cleaning up their environment/bridge/store registrations without any explicit teardown.

Key changes:

• useTangentSessionTabs now accepts a sessionId and stores tab state in a bySession map keyed by session ID. Switching sessions automatically restores the correct tab state. resetTabs is replaced by dropSession, which removes a specific session's entry from the map.
• TangentProjectContext introduces workareaTabsBySessionRef and workareaActiveBySessionRef as the per-session source of truth for workarea tabs. The existing workareaTabs state and refs mirror only the active session. On session switch, the incoming session's tabs are restored from these maps rather than wiping everything.
• setSessionTabs and setSessionActiveTabId helpers ensure all tab mutations are written to both the per-session maps and the active-session mirror atomically.
• activeSessionId is derived before useTangentSessionTabs is called so the hook always receives the correct session ID.
• syncActiveTabRefs extracts the ref-sync logic shared between the session-switch restore path and the useEffect that tracks active tab changes. It also improves the lastEditorTabIdRef fallback: if the last known editor tab is still open it is kept; otherwise the newest pipeline tab is used.
• When an empty session is discarded, its entries are removed from both per-session maps and dropSession is called to clean up the tab hook's state.

Screen Recording 2026-09-18 at 12.34.48 PM.mov (uploaded via Graphite)

Test Instructions

1. Open a Tangent project with multiple attached sessions.
2. Open sub-agent tabs and workarea tabs in session A.
3. Switch to session B — verify it starts with a clean slate.
4. Open different tabs in session B.
5. Switch back to session A — verify its previously open tabs are restored exactly as left.
6. Discard an empty session and confirm its tab state is fully cleaned up with no lingering entries.
7. Verify that switching sessions does not leave orphaned environment, bridge, or store registrations.

[github-actions]

🎩 Preview

A preview build has been created at: 09-16-feat_tangent_-_keep_track_of_open_tabs_while_switching_sessions_inside_project/69346a0

[maxy-shpfy]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: create a project from a tile in the grid #2735 : 3 dependent PRs (#2736 , #2737 , #2738 )
• feat: projects dashboard #2734
• chore: tangent - annotate runs submission to projectid #2758
• fix: tangent - proper debug sequence system prompt #2757
• fix: tangent - quick run button from mini content window #2744
• fix: tangent - remote agent send delta immediately #2743
• fix: tangent - mint proper authorization with tangent #2742
• feat: tangent - debug in tangent sequence #2741
• chore: add autolayout tool #2740
• feat: tangent - resource management via document entity #2733
• feat: tangent - interactive chips in tangent chat #2732
• chore: save id stack to replay ids on reloads #2731
• refactor: tangent - migrate tangetproectcontext onto mobx #2730
• feat: tangent - keep track of open tabs while switching sessions inside project #2729 👈 (View in Graphite)
• feat: tangent - remote agent for runview and editor #2728
• refactor: tangent - structured workarea target #2727
• feat: tangent - embedded pipeline run view #2726
• feat: tangent - embedded pipeline editor #2725
• feat: tangent - dynamic workarea remote tools #2724
• feat: tangent - resources management in project view #2723
• feat: tangent project view page #2722
• feat: workspaces and projects service #2720
• chore: api client for workspaces and projects #2719
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.