perf(sessions): extend transcript eviction grace from 20s to 3min

packages/core/src/sessions/sessionEventResidency.test.ts

@@ -1,11 +1,14 @@
 import type { AgentSession, SessionStatus } from "@posthog/shared";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
-import { SessionService, type SessionServiceDeps } from "./sessionService";
+import {
+  SESSION_EVENT_EVICT_GRACE_MS as GRACE_MS,
+  SessionService,
+  type SessionServiceDeps,
+} from "./sessionService";
 import { sessionStore, sessionStoreSetters } from "./sessionStore";
 
 const RUN = "run-res";
 const TASK = "task-res";
-const GRACE_MS = 20_000;
 
 const LOG_LINE = JSON.stringify({
   type: "notification",

packages/core/src/sessions/sessionService.ts

@@ -126,9 +126,10 @@ const SESSION_EVENT_FLUSH_MS = 16;
  * A backgrounded session's transcript is freed this long after it stops being
  * viewed, and reloaded from disk on return. Only disconnected (idle, no live
  * subscription) sessions are eligible, so no streamed event can append to an
- * evicted transcript.
+ * evicted transcript. Generous on purpose: reloading means a full log read and
+ * re-parse, so flipping between tasks within a few minutes must stay free.
  */
-const SESSION_EVENT_EVICT_GRACE_MS = 20_000;
+export const SESSION_EVENT_EVICT_GRACE_MS = 180_000;
 /**
  * On open, paint the last this-many bytes of the log immediately so a big
  * transcript shows its latest turns in tens of ms, while the authoritative