Cache definitive non-membership answers as long as confirmations

A merged PR's commit set is frozen, so a 200 response that lacks the PR is
as immutable as one that contains it — only errors (unpushed commits,
network) warrant the short retry TTL. Keeps a reused-branch worktree with
the checks panel open from re-probing GitHub every five minutes. Also adds
the review-requested why comments on the fallback gate and the preservation
seam.

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

src/main/github/client.test.ts

@@ -821,6 +821,15 @@ describe('getPRForBranch', () => {
     const pr = await getPRForBranch('/repo-root', 'fix-hibernation-wake')
 
     expect(pr).toBeNull()
+    expect(ghExecFileAsyncMock).toHaveBeenCalledTimes(3)
+
+    // A definitive "not part of this PR" answer is immutable for a merged PR:
+    // repeated polls must not re-probe GitHub.
+    mockMergedBranchPRLookupBehindHead()
+    const second = await getPRForBranch('/repo-root', 'fix-hibernation-wake')
+
+    expect(second).toBeNull()
+    expect(ghExecFileAsyncMock).toHaveBeenCalledTimes(5)
   })
 
   it('keeps hiding a merged branch PR when the commit membership probe fails', async () => {

src/main/github/client.ts

@@ -2950,6 +2950,10 @@ export async function getPRForBranchOutcome(
       explicitCurrentHeadOid !== null &&
       shouldHideMergedImplicitPR(data, linkedPRNumber, explicitCurrentHeadOid) &&
       !(await mergedPRContainsHead(data, dataRepo, explicitCurrentHeadOid))
+    // Why no lazy-HEAD re-check on preservation: fallback numbers come from
+    // callers that already gated them on head equality or confirmed
+    // containment; re-hiding against the main-repo HEAD would blank
+    // deleted-head merged PRs that are deliberately kept visible.
     const shouldPreserveMergedFallback =
       !explicitHeadHidesMergedImplicitPR &&
       (fallbackConfirmedMergedBranch || options.acceptMergedFallbackPR === true)

src/main/github/merged-pr-commit-membership.ts

@@ -4,12 +4,13 @@ import type { OwnerRepo } from './github-repository-identity'
 
 type GhExecOptions = Parameters<typeof ghExecFileAsync>[1]
 
-// Why: a merged PR's commit set is immutable, so confirmations never expire in
-// practice; the TTLs only bound memory and let "commit not on GitHub yet"
-// answers retry after a future push.
+// Why: a merged PR's commit set is immutable, so definitive answers — member
+// or not — never change; that TTL only bounds memory. Errors (a commit not on
+// GitHub yet, network) get a short TTL so a future push can flip the answer
+// without the checks-panel poll re-probing every cycle.
 const MEMBERSHIP_CACHE_MAX_ENTRIES = 200
-const MEMBERSHIP_CONFIRMED_TTL_MS = 6 * 60 * 60 * 1000
-const MEMBERSHIP_NEGATIVE_TTL_MS = 5 * 60 * 1000
+const MEMBERSHIP_DEFINITIVE_TTL_MS = 6 * 60 * 60 * 1000
+const MEMBERSHIP_ERROR_TTL_MS = 5 * 60 * 1000
 
 const membershipCache = new Map<string, { value: boolean; expiresAt: number }>()
 
@@ -80,7 +81,7 @@ export async function isCommitPartOfMergedPR(args: {
       )
     membershipCache.set(cacheKey, {
       value,
-      expiresAt: now + (value ? MEMBERSHIP_CONFIRMED_TTL_MS : MEMBERSHIP_NEGATIVE_TTL_MS)
+      expiresAt: now + MEMBERSHIP_DEFINITIVE_TTL_MS
     })
     return value
   } catch {
@@ -88,7 +89,7 @@ export async function isCommitPartOfMergedPR(args: {
     // definitive "new local work" today, but a push can change it; retry later.
     membershipCache.set(cacheKey, {
       value: false,
-      expiresAt: now + MEMBERSHIP_NEGATIVE_TTL_MS
+      expiresAt: now + MEMBERSHIP_ERROR_TTL_MS
     })
     return false
   }

src/renderer/src/store/slices/github.ts

@@ -1038,6 +1038,9 @@ function buildPRRefreshCandidate(
     true
   )
   const cachedFallbackPRNumber = cachedPR?.number ?? null
+  // Why: a merged PR stays a valid fallback when the worktree sits on its head
+  // or on a commit main confirmed as part of the PR; anything else means the
+  // branch moved on and the number must not resurrect the old merged PR.
   const cachedMergedPRMovedPastHead =
     worktree.linkedPR == null &&
     cachedPR?.state === 'merged' &&