fix(frontend): dedupe EventSource — share one session SSE stream

[lucastononro]

Summary

• HomePage.connectSSE and useNotebookSSE each opened an independent EventSource to the identical /api/sessions/{id}/stream endpoint, doubling server fan-out and parsing every event twice.
• Introduces SSEStreamContext (src/lib/SSEStreamContext.tsx): a publish/subscribe bus exposing the single page-level stream. connectSSE republishes every parsed message to it; the notebook now subscribes instead of opening a second connection.

Changes

• New src/lib/SSEStreamContext.tsx — SSEStreamProvider + useSSEStream() (subscribe/publish pair).
• src/app/page.tsx — split the default export into HomePageContent (unchanged logic) wrapped by a new HomePage that mounts SSEStreamProvider; connectSSE's onmessage now calls publish(event) for every parsed message, in addition to its existing switch handling.
• src/lib/notebook/useNotebookSSE.ts — no longer opens its own EventSource; subscribes to the shared stream via useSSEStream() and keeps the exact same notebook.* event filtering/dispatch logic.

Test plan

Existing (must still work)

• Streaming chat: send a message, confirm assistant tokens stream in as before (tool_start/tool_end, subagent cards, metrics, canvas/report auto-open, tasks card) — connectSSE's switch is untouched.
• Live metrics tab updates as metric/metrics_batch events arrive.
• File/image/PDF viewing in the workspace canvas.
• Reload a session mid-run — chat/canvas/metrics restore and SSE reconnects.

New

• Open a notebook file (.ipynb) while an agent run is active — confirm cell outputs/kernel state/structure-changed events still update live, now sourced from the shared stream (verify in devtools Network tab that only one EventSource connection to /api/sessions/{id}/stream is open, not two).
• Switch sessions/experiments while a notebook is open — no duplicate/stale subscriptions (each useNotebookSSE call cleans up via its returned unsubscribe on unmount/dep change).

Closes #100

🤖 Generated with Claude Code

Greptile Summary

This PR eliminates duplicate EventSource connections to /api/sessions/{id}/stream by introducing a lightweight pub/sub bus (SSEStreamContext). connectSSE in page.tsx feeds the bus via publish, and useNotebookSSE subscribes to it instead of opening its own connection. A session-ID guard (eventSessionId !== sessionId) prevents cross-session event bleed during session switches, and new vitest tests cover fan-out, session isolation, and notebook filtering.

• SSEStreamContext.tsx: New stateless context holding stable subscribe/publish callbacks backed by a Set<SSEListener> ref \u2014 no re-renders triggered on consumers.
• page.tsx: Splits the default export into SSEStreamProvider \u2192 HomePageContent; connectSSE.onmessage calls publish(sid, event) before its existing switch.
• useNotebookSSE.ts: Removes its own EventSource; subscribes to the shared bus with a sessionId guard and stable handler/filter refs.

Confidence Score: 4/5

Safe to merge after addressing listener error isolation in publish — as-is, a throwing subscriber silently aborts all chat/metrics/UI updates for that event.

The publish call sits inside connectSSE's try/catch block before the switch that drives every UI update. If any listener throws, the catch block swallows it and the switch never runs for that event — chat messages, metrics, and file-tree updates would be silently lost. The fix is small and localised, but it guards a critical code path.

frontend/src/lib/SSEStreamContext.tsx (publish forEach) and the call site in frontend/src/app/page.tsx where publish precedes the switch inside a swallowing catch block.

Important Files Changed

Filename	Overview
frontend/src/lib/SSEStreamContext.tsx	New pub/sub context; the publish forEach has no error isolation, allowing a listener exception to propagate to connectSSE's catch block and abort the switch for that event.
frontend/src/app/page.tsx	Wraps HomePageContent in SSEStreamProvider; publish(sid, event) is called before the switch inside a catch-all try block — listener errors will silently skip all UI updates for that event.
frontend/src/lib/notebook/useNotebookSSE.ts	Correctly migrated from its own EventSource to subscribing the shared bus; includes an eventSessionId guard and stable handler/filter refs.
frontend/src/lib/notebook/useNotebookSSE.test.tsx	Good coverage of session-isolation, notebook filtering, disabled state, and fan-out; module-level bus pattern is standard for vitest sequential execution.
frontend/vitest.config.ts	New vitest config with jsdom environment, React plugin for JSX, and correct @ alias; straightforward and correct.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant Backend as Backend SSE
    participant ES as EventSource (single)
    participant CSE as connectSSE.onmessage
    participant Bus as SSEStreamContext (pub/sub)
    participant NB as useNotebookSSE

    Backend->>ES: SSE event
    ES->>CSE: onmessage(e)
    CSE->>CSE: JSON.parse(e.data)
    CSE->>Bus: publish(sid, event)
    Bus->>NB: listener(sid, event)
    NB->>NB: sessionId guard check
    NB->>NB: "notebook.* filter + dispatch"
    CSE->>CSE: switch(event.type) — UI updates

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant Backend as Backend SSE
    participant ES as EventSource (single)
    participant CSE as connectSSE.onmessage
    participant Bus as SSEStreamContext (pub/sub)
    participant NB as useNotebookSSE

    Backend->>ES: SSE event
    ES->>CSE: onmessage(e)
    CSE->>CSE: JSON.parse(e.data)
    CSE->>Bus: publish(sid, event)
    Bus->>NB: listener(sid, event)
    NB->>NB: sessionId guard check
    NB->>NB: "notebook.* filter + dispatch"
    CSE->>CSE: switch(event.type) — UI updates

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[greptile-apps]

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[lucastononro]

Merged into integration branch staging-v0.0.5 (see the STAGING-v0.0.5.md ledger on that branch for merge order, Greptile follow-ups and test results). These changes will land on main via the staging merge — closing to clear the queue.