fix(terminal): demote un-parkable worktrees the force-park lever spared (C1 review #4)

The last-active exemption means a single hidden un-parkable worktree never
force-parks — and slice C previously only targeted exempt-tab worktrees, so
its panes held full scrollback forever. Demotion now also covers un-parkable
non-exempt worktrees past the retention TTL that are absent from the
force-parked set (last-active spared, or slice B switched off). Membership
stays time-monotone for fixed inputs; covered by new idempotence/monotone
selector tests.

Co-authored-by: Orca <help@stably.ai>

src/renderer/src/components/terminal-pane/terminal-hidden-worktree-retention.test.ts

@@ -233,6 +233,61 @@ describe('selectScrollbackDemotedTerminalWorktrees', () => {
     ).toEqual(new Set(['wt-a', 'wt-b']))
   })
 
+  it('demotes un-parkable non-exempt worktrees the force-park lever spared, past the TTL', () => {
+    // wt-solo is the only candidate → last-active exempt from force-park; its
+    // panes stay fully mounted forever unless demotion bounds them.
+    const solo = [exemptCandidate('wt-solo', aged, { hasEvictionExemptTab: false })]
+    expect(selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees: solo })).toEqual(
+      new Set(['wt-solo'])
+    )
+    // With slice B off nothing force-parks; every un-parkable worktree past
+    // the TTL demotes (C bounds independently).
+    const pair = [
+      exemptCandidate('wt-a', aged - 1, { hasEvictionExemptTab: false }),
+      exemptCandidate('wt-b', aged, { hasEvictionExemptTab: false })
+    ]
+    expect(
+      selectScrollbackDemotedTerminalWorktrees({
+        ...base,
+        worktrees: pair,
+        retentionBudgetEnabled: false
+      })
+    ).toEqual(new Set(['wt-a', 'wt-b']))
+    // With slice B on, wt-a force-parks (unmounted — no demotion needed) and
+    // only the spared last-active wt-b demotes.
+    expect(selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees: pair })).toEqual(
+      new Set(['wt-b'])
+    )
+    // Ordinary-parking-covered worktrees are bounded by the warm cap already.
+    const covered = [
+      exemptCandidate('wt-covered', aged, {
+        hasEvictionExemptTab: false,
+        ordinaryParkingCovers: true
+      })
+    ]
+    expect(selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees: covered })).toEqual(
+      new Set()
+    )
+  })
+
+  it('is idempotent and only grows as time advances (flip-loop dwell)', () => {
+    const worktrees = [
+      exemptCandidate('wt-exempt', aged),
+      exemptCandidate('wt-force-parked', aged, { hasEvictionExemptTab: false }),
+      exemptCandidate('wt-last-active', nowMs - 60_000, { hasEvictionExemptTab: false })
+    ]
+    const first = selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees })
+    expect(selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees })).toEqual(first)
+    // Why: with unchanged inputs, a later evaluation may only ADD members —
+    // a verdict that oscillates with time is the React-#185 ingredient.
+    for (const laterMs of [nowMs + 1_000, nowMs + TERMINAL_HIDDEN_WORKTREE_RETENTION_TTL_MS]) {
+      const later = selectScrollbackDemotedTerminalWorktrees({ ...base, worktrees, nowMs: laterMs })
+      for (const id of first) {
+        expect(later.has(id)).toBe(true)
+      }
+    }
+  })
+
   it('honors the retentionTtlMs override and stays time-monotone', () => {
     const worktrees = [exemptCandidate('wt-1', nowMs - 500)]
     expect(

src/renderer/src/components/terminal-pane/terminal-hidden-worktree-retention.ts

@@ -96,11 +96,14 @@ export function selectRetentionForceParkedTerminalWorktrees(
 
 /**
  * Scrollback demotion targets the hidden panes that stay mounted past the
- * retention deadlines: eviction-exempt tabs' worktrees (their panes survive
- * force-park via the per-tab exclusion) demote past the TTL — or immediately
- * when their worktree force-parks under the count budget, since the exempt
- * panes are then the only ones left mounted. Time-monotone for fixed inputs:
- * the force-parked set and the past-TTL set only ever grow with nowMs, so
+ * retention deadlines: (a) eviction-exempt tabs' worktrees (their panes
+ * survive force-park via the per-tab exclusion) demote past the TTL — or
+ * immediately when their worktree force-parks under the count budget, since
+ * the exempt panes are then the only ones left mounted; (b) un-parkable
+ * worktrees the force-park lever spared (the last-active exemption, or slice
+ * B switched off) demote past the TTL instead of holding full scrollback
+ * forever. Time-monotone for fixed inputs: the force-parked and past-TTL sets
+ * only ever grow with nowMs and the last-active pick is time-independent, so
  * membership only grows until a reveal resets hiddenSince — no oscillation
  * surface.
  */
@@ -128,12 +131,20 @@ export function selectScrollbackDemotedTerminalWorktrees(
       worktree.isVisible ||
       worktree.shouldMeasureHiddenWorktree ||
       worktree.hasActivityTerminalPortal ||
-      !worktree.hasEvictionExemptTab ||
       worktree.hasPendingSpawnWork
     ) {
       continue
     }
-    if (args.nowMs - worktree.hiddenSinceMs >= ttlMs || forceParked.has(worktree.worktreeId)) {
+    const pastTtl = args.nowMs - worktree.hiddenSinceMs >= ttlMs
+    if (worktree.hasEvictionExemptTab) {
+      if (pastTtl || forceParked.has(worktree.worktreeId)) {
+        demoted.add(worktree.worktreeId)
+      }
+      continue
+    }
+    // Force-park candidates the lever spared stay fully mounted, so demotion
+    // is their only bound; force-parked ones already unmounted.
+    if (!worktree.ordinaryParkingCovers && pastTtl && !forceParked.has(worktree.worktreeId)) {
       demoted.add(worktree.worktreeId)
     }
   }