fix(github): validate implicit default-branch PR repository identity

src/main/github/client-merge-queue-auto-merge.test.ts

@@ -110,7 +110,7 @@ describe('GitHub GraphQL rate-limit guard', () => {
         '--repo',
         'stablyai/orca',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root', host: 'github.com' }
     )

src/main/github/client-pr-branch-discovery.test.ts

@@ -188,7 +188,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )

src/main/github/client-pr-fallback-number.test.ts

@@ -126,7 +126,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -162,7 +162,7 @@ describe('getPRForBranch', () => {
         '--limit',
         '1',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -308,7 +308,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'stablyai/orca',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -381,7 +381,7 @@ describe('getPRForBranch', () => {
         '--limit',
         '1',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -507,7 +507,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )

src/main/github/client-pr-linked-lookup.test.ts

@@ -80,7 +80,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'stablyai/orca',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -93,7 +93,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'fork/orca',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -132,7 +132,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -411,7 +411,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -502,7 +502,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -543,7 +543,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )
@@ -579,7 +579,7 @@ describe('getPRForBranch', () => {
         'view',
         '77',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/non-github-repo' }
     )
@@ -618,7 +618,7 @@ describe('getPRForBranch', () => {
         'view',
         'feature/test',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/non-github-repo' }
     )
@@ -658,7 +658,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )

src/main/github/client-tracked-upstream-default-branch.test.ts

@@ -1,9 +1,4 @@
-/*
- * Issue #26948: a branch created with `git switch -c feature/x origin/develop`
- * tracks the default branch. Its PR lookup misses, falls back to the tracked
- * upstream, and attaches the newest PR whose head is `develop` (an unrelated
- * develop -> release PR), then caches that number so the link survives restarts.
- */
+// #26948: tracking a base branch must not attach its integration PR to a feature branch.
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 import type * as GithubApiRepositoryModule from './github-api-repository'
 import type * as GitHubEnterpriseRepositoryModule from './github-enterprise-repository'
@@ -35,6 +30,8 @@ vi.mock('./github-api-repository', async (importOriginal) =>
 import { getPRForBranch } from './client/lookup/get-pr-for-branch'
 import { getPRForBranchOutcome } from './client/lookup/pr-for-branch-outcome'
 import { resetPRForBranchMocks } from './client-test-harness'
+import type { RestPullRequest } from './client/lookup/pull-request-lookup-data'
+import type { OwnerRepo } from './gh-utils'
 
 const {
   ghExecFileAsyncMock,
@@ -45,6 +42,8 @@ const {
 } = clientMocks
 
 const ACME = { owner: 'acme', repo: 'widgets' }
+const UPSTREAM = { owner: 'stablyai', repo: 'widgets' }
+const CONTRIBUTOR = { owner: 'contributor', repo: 'widgets' }
 
 /** remoteHeads maps a remote to the branch its `refs/remotes/<remote>/HEAD` points at. */
 function primeGit(
@@ -71,43 +70,59 @@ function primeGit(
   })
 }
 
-function restPR(headOwnerRef: string): Record<string, unknown> {
+function restPR(
+  headRefName = 'develop',
+  headRepo: OwnerRepo = ACME,
+  prRepo: OwnerRepo = ACME
+): RestPullRequest {
   return {
     number: 7,
     title: 'Release develop',
     state: 'closed',
     merged_at: null,
-    html_url: 'https://github.com/acme/widgets/pull/7',
+    html_url: `https://github.com/${prRepo.owner}/${prRepo.repo}/pull/7`,
     updated_at: '2025-01-01T00:00:00Z',
     draft: false,
     mergeable: null,
     base: { ref: 'release', sha: 'release-oid' },
-    head: { ref: headOwnerRef, sha: 'old-develop-oid' }
+    head: {
+      ref: headRefName,
+      sha: 'old-develop-oid',
+      repo: { name: headRepo.repo, owner: { login: headRepo.owner } }
+    }
   }
 }
 
-/** Branch-list responses keyed by the `head=` filter; every other gh call fails. */
-function primeGh(listsByHead: Record<string, Record<string, unknown>[]>): void {
+function primeGh(
+  listsByHead: Record<string, RestPullRequest[]>,
+  exactPR: RestPullRequest = Object.values(listsByHead).flat()[0] ?? restPR()
+): void {
+  const prsByNumber = new Map(
+    [...Object.values(listsByHead).flat(), exactPR].map((pr) => [pr.number, pr])
+  )
   ghExecFileAsyncMock.mockImplementation(async (args: string[]) => {
     const head = args[1]?.match(/pulls\?head=([^&]+)/)?.[1]
     if (args[0] === 'api' && head) {
       return { stdout: JSON.stringify(listsByHead[decodeURIComponent(head)] ?? []) }
     }
-    if (args[0] === 'pr' && args[1] === 'view' && args[2] === '7') {
+    const exact = prsByNumber.get(Number(args[2]))
+    if (args[0] === 'pr' && args[1] === 'view' && exact) {
       return {
         stdout: JSON.stringify({
-          number: 7,
-          title: 'Release develop',
-          state: 'CLOSED',
-          url: 'https://github.com/acme/widgets/pull/7',
+          number: exact.number,
+          title: exact.title,
+          state: exact.merged_at ? 'MERGED' : exact.state.toUpperCase(),
+          url: exact.html_url,
           statusCheckRollup: [],
-          updatedAt: '2025-01-01T00:00:00Z',
-          isDraft: false,
+          updatedAt: exact.updated_at,
+          isDraft: exact.draft,
           mergeable: 'UNKNOWN',
-          baseRefName: 'release',
-          headRefName: 'develop',
-          baseRefOid: 'release-oid',
-          headRefOid: 'old-develop-oid'
+          baseRefName: exact.base?.ref,
+          headRefName: exact.head?.ref,
+          baseRefOid: exact.base?.sha,
+          headRefOid: exact.head?.sha,
+          headRepositoryOwner: exact.head?.repo?.owner,
+          headRepository: exact.head?.repo ? { name: exact.head.repo.name } : null
         })
       }
     }
@@ -117,7 +132,6 @@ function primeGh(listsByHead: Record<string, Record<string, unknown>[]>): void {
 
 /** A fork clone: `origin` is the fork, `upstream` is the repo PRs target. */
 function primeForkWithUpstream(): void {
-  const UPSTREAM = { owner: 'stablyai', repo: 'widgets' }
   resolvePRRepositoryCandidatesMock.mockResolvedValue({
     candidates: [UPSTREAM, ACME],
     headRepo: ACME
@@ -153,8 +167,8 @@ describe('issue #26948: a branch tracking the default branch', () => {
 
   it('skips the default branch on a second remote that PRs target (fork checkout off upstream)', async () => {
     primeForkWithUpstream()
-    primeGit('refs/remotes/upstream/develop')
-    primeGh({ 'stablyai:develop': [restPR('develop')] })
+    primeGit('refs/remotes/upstream/develop', { origin: 'develop', upstream: 'develop' })
+    primeGh({ 'stablyai:develop': [restPR('develop', UPSTREAM, UPSTREAM)] })
 
     const pr = await getPRForBranch('/repo-root', 'feature/my-change')
 
@@ -164,7 +178,7 @@ describe('issue #26948: a branch tracking the default branch', () => {
   it("uses the tracked remote's own default branch when it differs from origin's", async () => {
     primeForkWithUpstream()
     primeGit('refs/remotes/upstream/develop', { origin: 'main', upstream: 'develop' })
-    primeGh({ 'stablyai:develop': [restPR('develop')] })
+    primeGh({ 'stablyai:develop': [restPR('develop', UPSTREAM, UPSTREAM)] })
 
     const pr = await getPRForBranch('/repo-root', 'feature/my-change')
 
@@ -174,7 +188,7 @@ describe('issue #26948: a branch tracking the default branch', () => {
   it("keeps a PR headed by a tracked branch that is only origin's default", async () => {
     primeForkWithUpstream()
     primeGit('refs/remotes/upstream/develop', { origin: 'develop', upstream: 'main' })
-    primeGh({ 'stablyai:develop': [restPR('develop')] })
+    primeGh({ 'stablyai:develop': [restPR('develop', UPSTREAM, UPSTREAM)] })
 
     const pr = await getPRForBranch('/repo-root', 'feature/my-change')
 
@@ -186,10 +200,100 @@ describe('issue #26948: a branch tracking the default branch', () => {
       remoteName === 'contributor' ? { owner: 'contributor', repo: 'widgets' } : ACME
     )
     primeGit('refs/remotes/contributor/develop')
-    primeGh({ 'contributor:develop': [{ ...restPR('develop'), state: 'open' }] })
+    primeGh({ 'contributor:develop': [{ ...restPR('develop', CONTRIBUTOR), state: 'open' }] })
 
     const pr = await getPRForBranch('/repo-root', 'feature/my-change')
 
     expect(pr).toMatchObject({ number: 7, headRepo: { owner: 'contributor', repo: 'widgets' } })
   })
+
+  it('preserves a cached contributor PR sharing the tracked default branch name', async () => {
+    primeGit('refs/remotes/origin/develop')
+    primeGh({}, { ...restPR('develop', CONTRIBUTOR), state: 'open' })
+    const pr = await getPRForBranch('/repo-root', 'feature/my-change', null, null, 7)
+    expect(pr).toMatchObject({ number: 7, headRepo: CONTRIBUTOR })
+  })
+
+  it('preserves a PR from the origin fork default branch into upstream', async () => {
+    primeForkWithUpstream()
+    primeGit('refs/remotes/origin/develop')
+    primeGh({ 'acme:develop': [{ ...restPR('develop', ACME, UPSTREAM), state: 'open' }] })
+    const pr = await getPRForBranch('/repo-root', 'feature/my-change')
+    expect(pr).toMatchObject({ number: 7, headRepo: ACME, prRepo: UPSTREAM })
+  })
+
+  it('clears an old cached integration PR after upstream tracking is removed', async () => {
+    primeGit('')
+    primeGh({})
+    expect(
+      (await getPRForBranchOutcome('/repo-root', 'feature/my-change', null, null, 7)).kind
+    ).toBe('no-pr')
+  })
+
+  it('tries the cached feature PR after rejecting an unrelated upstream result', async () => {
+    primeGit('refs/remotes/origin/develop')
+    primeGh(
+      { 'acme:develop': [restPR()] },
+      { ...restPR('feature/my-change'), number: 8, state: 'open' }
+    )
+    const pr = await getPRForBranch('/repo-root', 'feature/my-change', null, null, 8)
+    expect(pr).toMatchObject({ number: 8, headRefName: 'feature/my-change' })
+  })
+
+  it('does not borrow origin default when upstream HEAD is unknown', async () => {
+    primeForkWithUpstream()
+    primeGit('refs/remotes/upstream/develop', { origin: 'develop' })
+    primeGh({ 'stablyai:develop': [restPR('develop', UPSTREAM, UPSTREAM)] })
+    expect(await getPRForBranch('/repo-root', 'feature/my-change')).toMatchObject({ number: 7 })
+  })
+
+  it('uses PR head default metadata when upstream has no recorded HEAD', async () => {
+    primeForkWithUpstream()
+    primeGit('refs/remotes/upstream/develop', { origin: 'main' })
+    const pr = restPR('develop', UPSTREAM, UPSTREAM)
+    primeGh({
+      'stablyai:develop': [
+        {
+          ...pr,
+          head: {
+            ...pr.head,
+            repo: { name: 'widgets', owner: { login: 'stablyai' }, default_branch: 'develop' }
+          }
+        }
+      ]
+    })
+    expect(await getPRForBranch('/repo-root', 'feature/my-change')).toBeNull()
+  })
+
+  it('preserves explicit links to integration PRs', async () => {
+    primeGit('refs/remotes/origin/develop')
+    primeGh({})
+    expect(await getPRForBranch('/repo-root', 'feature/my-change', 7)).toMatchObject({ number: 7 })
+    expect(gitExecFileAsyncMock).not.toHaveBeenCalled()
+  })
+
+  it('preserves cached review context while detached', async () => {
+    primeGh({})
+    expect(await getPRForBranch('/repo-root', '', null, null, 7)).toMatchObject({ number: 7 })
+    expect(gitExecFileAsyncMock).not.toHaveBeenCalled()
+  })
+
+  it('does not infer a head repository for a cached PR with unavailable fork metadata', async () => {
+    primeGit('refs/remotes/origin/develop')
+    const pr = restPR()
+    primeGh({}, { ...pr, head: { ref: 'develop', sha: 'deleted-fork-oid', repo: null } })
+    expect(await getPRForBranch('/repo-root', 'feature/my-change', null, null, 7)).toMatchObject({
+      number: 7
+    })
+  })
+
+  it('does not fetch the rejected integration PR twice when its number is cached', async () => {
+    primeGit('refs/remotes/origin/develop')
+    primeGh({ 'acme:develop': [restPR()] })
+    expect(await getPRForBranch('/repo-root', 'feature/my-change', null, null, 7)).toBeNull()
+    const exactCalls = ghExecFileAsyncMock.mock.calls.filter(
+      ([args]) => args[0] === 'pr' && args[1] === 'view'
+    )
+    expect(exactCalls).toHaveLength(1)
+  })
 })

src/main/github/client-tracked-upstream-snapshot.test.ts

@@ -113,7 +113,7 @@ describe('getPRForBranch', () => {
         '--repo',
         'acme/widgets',
         '--json',
-        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+        'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
       ],
       { cwd: '/repo-root' }
     )

src/main/github/client/lookup/branch-lookup-resolution.ts

@@ -17,6 +17,8 @@ import {
   isMergedImplicitPR,
   shouldHideMergedImplicitPR,
   getCurrentHeadOid,
+  pullRequestHeadRepository,
+  linkedMergedPRDivergedHeadOid,
   type PullRequestLookupData,
   type GitHubPRBranchLookupOptions
 } from './pull-request-lookup-data'
@@ -29,7 +31,7 @@ import {
   type TrackedUpstreamBranch
 } from './tracked-upstream-cache'
 import { getTrackedUpstreamBranch } from './tracked-upstream-branch'
-import { isTrackedUpstreamDefaultBranch } from './tracked-upstream-default-branch'
+import { shouldHideImplicitDefaultBranchPR } from './implicit-default-branch-pr'
 import { PR_BRANCH_LOOKUP_BUCKETS } from './pr-lookup-rate-limit'
 import type { HostedReviewLocalGitOptions } from './../github-exec-scope'
 export async function resolvePRForBranchOutcome(input: {
@@ -105,29 +107,6 @@ export async function resolvePRForBranchOutcome(input: {
     }
     return membership
   }
-  const recordLinkedMergedPRDivergence = async (
-    candidate: PullRequestLookupData | null,
-    candidateRepo: OwnerRepo | null
-  ): Promise<void> => {
-    if (
-      typeof linkedPRNumber !== 'number' ||
-      !candidate ||
-      mapPRState(candidate.state, candidate.isDraft) !== 'merged' ||
-      explicitCurrentHeadOid === null ||
-      candidate.headRefOid === explicitCurrentHeadOid
-    ) {
-      return
-    }
-    const membership = await mergedPRContainsHead(
-      candidate,
-      candidateRepo ?? ownerRepoFromPullRequestUrl(candidate.url),
-      explicitCurrentHeadOid
-    )
-    if (membership === 'not-contained') {
-      // explicitCurrentHeadOid is non-null here (guarded above); record the exact diverged head so consumers clear only that worktree.
-      headDivergedFromMergedPRAtOid = explicitCurrentHeadOid
-    }
-  }
   const hideMergedImplicitPR = async (
     candidate: PullRequestLookupData | null,
     candidateRepo: OwnerRepo | null
@@ -216,6 +195,29 @@ export async function resolvePRForBranchOutcome(input: {
       }
     }
   }
+  let rejectedDefaultPRNumber: number | null = null
+  const hideUnrelatedDefaultPR = async (): Promise<void> => {
+    if (
+      data &&
+      (await shouldHideImplicitDefaultBranchPR({
+        data,
+        prRepo: dataRepo,
+        originHeadRepo: headRepo,
+        trackedUpstream: retriedUpstream,
+        branchName,
+        linkedPRNumber,
+        repoPath,
+        connectionId,
+        localGitOptions
+      }))
+    ) {
+      rejectedDefaultPRNumber = data.number
+      data = null
+      dataRepo = null
+      dataHeadRepo = headRepo
+    }
+  }
+  await hideUnrelatedDefaultPR()
   let mergedBranchLookupNumber: number | null = null
   if (await hideMergedImplicitPR(data, dataRepo)) {
     mergedBranchLookupNumber = data?.number ?? null
@@ -223,7 +225,12 @@ export async function resolvePRForBranchOutcome(input: {
     dataRepo = null
     dataHeadRepo = headRepo
   }
-  if (!data && typeof linkedPRNumber !== 'number' && typeof fallbackPRNumber === 'number') {
+  if (
+    !data &&
+    typeof linkedPRNumber !== 'number' &&
+    typeof fallbackPRNumber === 'number' &&
+    fallbackPRNumber !== rejectedDefaultPRNumber
+  ) {
     usedExactNumberLookup = true
     const fallbackLookup = await lookupPRByNumber({
       candidates,
@@ -234,30 +241,26 @@ export async function resolvePRForBranchOutcome(input: {
     data = fallbackLookup.data
     dataRepo = fallbackLookup.dataRepo
   }
-  // Why: covers the cached fallback number too, so a link stored before #26948 heals.
-  if (
-    data &&
-    retriedUpstream &&
-    data.headRefName === retriedUpstream.branch.branchName &&
-    (await isTrackedUpstreamDefaultBranch({
-      upstreamBranch: retriedUpstream.branch,
-      upstreamHeadRepo: retriedUpstream.headRepo,
-      candidates,
-      repoPath,
-      connectionId,
-      localGitOptions
-    }))
-  ) {
-    data = null
-    dataRepo = null
-  }
+  // Recheck exact fallbacks so links cached before #26948 heal without upstream tracking.
+  await hideUnrelatedDefaultPR()
   if (!data) {
     if (hasPendingBranchLookupError) {
       return prRefreshUpstreamError(pendingBranchLookupError)
     }
     return { kind: 'no-pr', fetchedAt: Date.now() }
   }
-  await recordLinkedMergedPRDivergence(data, dataRepo)
+  dataHeadRepo = pullRequestHeadRepository(data, dataRepo) ?? dataHeadRepo
+  headDivergedFromMergedPRAtOid = await linkedMergedPRDivergedHeadOid(
+    data,
+    linkedPRNumber,
+    explicitCurrentHeadOid,
+    (candidate, headOid) =>
+      mergedPRContainsHead(
+        candidate,
+        dataRepo ?? ownerRepoFromPullRequestUrl(candidate.url),
+        headOid
+      )
+  )
   const fallbackConfirmedMergedBranch =
     typeof fallbackPRNumber === 'number' &&
     mergedBranchLookupNumber === fallbackPRNumber &&

src/main/github/client/lookup/implicit-default-branch-pr.ts

@@ -0,0 +1,60 @@
+import type { OwnerRepo } from '../../gh-utils'
+import { githubRepoIdentityKey } from '../../../../shared/github/repository-identity-key'
+import {
+  getRemoteHeadBranchName,
+  getRepoDefaultBranchName
+} from '../../../source-control/repo-default-branch'
+import type { HostedReviewLocalGitOptions } from './../github-exec-scope'
+import type { TrackedUpstreamBranch } from './tracked-upstream-cache'
+import { pullRequestHeadRepository, type PullRequestLookupData } from './pull-request-lookup-data'
+
+export async function shouldHideImplicitDefaultBranchPR(input: {
+  data: PullRequestLookupData
+  prRepo: OwnerRepo | null
+  originHeadRepo: OwnerRepo | null
+  trackedUpstream: { branch: TrackedUpstreamBranch; headRepo: OwnerRepo } | null
+  branchName: string
+  linkedPRNumber?: number | null
+  repoPath: string
+  connectionId?: string | null
+  localGitOptions: HostedReviewLocalGitOptions
+}): Promise<boolean> {
+  const { data, branchName, prRepo } = input
+  if (
+    !branchName ||
+    !data.headRefName ||
+    data.headRefName === branchName ||
+    typeof input.linkedPRNumber === 'number'
+  ) {
+    return false
+  }
+  const headRepo = pullRequestHeadRepository(data, prRepo)
+  // Fork heads are real review branches even when their name is a default branch elsewhere.
+  if (!headRepo || !prRepo || githubRepoIdentityKey(headRepo) !== githubRepoIdentityKey(prRepo)) {
+    return false
+  }
+  if (data.headDefaultBranchName) {
+    return data.headRefName === data.headDefaultBranchName
+  }
+  const headRepoKey = githubRepoIdentityKey(headRepo)
+  const remoteName =
+    input.trackedUpstream && githubRepoIdentityKey(input.trackedUpstream.headRepo) === headRepoKey
+      ? input.trackedUpstream.branch.remoteName
+      : input.originHeadRepo && githubRepoIdentityKey(input.originHeadRepo) === headRepoKey
+        ? 'origin'
+        : null
+  if (!remoteName) {
+    return false
+  }
+  // An unknown upstream HEAD cannot borrow a different repository's origin default.
+  const defaultBranchName =
+    remoteName === 'origin'
+      ? await getRepoDefaultBranchName(input.repoPath, input.connectionId, input.localGitOptions)
+      : await getRemoteHeadBranchName(
+          input.repoPath,
+          remoteName,
+          input.connectionId,
+          input.localGitOptions
+        )
+  return defaultBranchName !== null && data.headRefName === defaultBranchName
+}

src/main/github/client/lookup/pull-request-lookup-data.ts

@@ -4,10 +4,11 @@ import type {
   PRMergeableState,
   PRReviewDecision
 } from '../../../../shared/github/pull-request-types'
-import { gitExecFileAsync } from '../../gh-utils'
+import { gitExecFileAsync, type OwnerRepo } from '../../gh-utils'
 import type { GitAdmissionTier } from '../../../git/command-runner/git-exec-options'
 import type { HostedReviewExecutionOptions } from '../../../source-control/hosted-review-git-options'
 import { mapPRState } from '../../mappers'
+import type { MergedPRCommitMembership } from '../../merged-pr-commit-membership'
 import {
   normalizePRMergeable,
   normalizeReviewDecision,
@@ -35,6 +36,9 @@ export type PullRequestLookupData = {
   headRefName?: string
   baseRefOid?: string
   headRefOid?: string
+  headRepositoryOwner?: { login?: string } | null
+  headRepository?: { name?: string } | null
+  headDefaultBranchName?: string
   stack?: GitHubPRStack
   stackMetadataChecked?: boolean
 }
@@ -51,7 +55,12 @@ export type RestPullRequest = {
   mergeable?: boolean | null
   mergeable_state?: string | null
   base?: { ref?: string; sha?: string }
-  head?: { ref?: string; sha?: string }
+  head?: {
+    ref?: string
+    sha?: string
+    repo?: { name?: string; owner?: { login?: string }; default_branch?: string } | null
+    user?: { login?: string }
+  }
   stack?: {
     number?: number
     position?: number
@@ -61,10 +70,10 @@ export type RestPullRequest = {
 }
 
 export const PR_LOOKUP_JSON_FIELDS =
-  'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid'
+  'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,reviewDecision,mergeStateStatus,autoMergeRequest,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
 
 export const PR_BRANCH_LIST_JSON_FIELDS =
-  'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,baseRefName,headRefName,baseRefOid,headRefOid'
+  'number,title,state,url,statusCheckRollup,updatedAt,isDraft,mergeable,baseRefName,headRefName,baseRefOid,headRefOid,headRepositoryOwner,headRepository'
 
 export type GitHubPRBranchLookupOptions = HostedReviewExecutionOptions & {
   acceptMergedFallbackPR?: boolean
@@ -118,11 +127,23 @@ export function mapRestPullRequest(pr: RestPullRequest): PullRequestLookupData {
     headRefName: pr.head?.ref,
     baseRefOid: pr.base?.sha,
     headRefOid: pr.head?.sha,
+    headRepositoryOwner: pr.head?.repo?.owner ?? pr.head?.user,
+    headRepository: pr.head?.repo,
+    headDefaultBranchName: pr.head?.repo?.default_branch,
     stackMetadataChecked: true,
     ...(stack ? { stack } : {})
   }
 }
 
+export function pullRequestHeadRepository(
+  data: PullRequestLookupData,
+  prRepo: OwnerRepo | null
+): OwnerRepo | null {
+  const owner = data.headRepositoryOwner?.login?.trim()
+  const repo = data.headRepository?.name?.trim()
+  return owner && repo ? { owner, repo, ...(prRepo?.host ? { host: prRepo.host } : {}) } : null
+}
+
 export function isMergedImplicitPR(
   data: PullRequestLookupData,
   linkedPRNumber?: number | null
@@ -143,6 +164,24 @@ export function shouldHideMergedImplicitPR(
   return !currentHeadOid || data.headRefOid !== currentHeadOid
 }
 
+export async function linkedMergedPRDivergedHeadOid(
+  data: PullRequestLookupData | null,
+  linkedPRNumber: number | null | undefined,
+  currentHeadOid: string | null,
+  containsHead: (data: PullRequestLookupData, headOid: string) => Promise<MergedPRCommitMembership>
+): Promise<string | null> {
+  if (
+    typeof linkedPRNumber !== 'number' ||
+    !data ||
+    mapPRState(data.state, data.isDraft) !== 'merged' ||
+    currentHeadOid === null ||
+    data.headRefOid === currentHeadOid
+  ) {
+    return null
+  }
+  return (await containsHead(data, currentHeadOid)) === 'not-contained' ? currentHeadOid : null
+}
+
 export function normalizePullRequestLookupData(data: PullRequestLookupData): PullRequestLookupData {
   return {
     ...data,

src/main/github/client/lookup/tracked-upstream-default-branch.ts

@@ -1,42 +0,0 @@
-import type { OwnerRepo } from '../../gh-utils'
-import { githubRepoIdentityKey } from '../../../../shared/github/repository-identity-key'
-import {
-  getRemoteHeadBranchName,
-  getRepoDefaultBranchName
-} from '../../../source-control/repo-default-branch'
-import type { HostedReviewLocalGitOptions } from './../github-exec-scope'
-import type { TrackedUpstreamBranch } from './tracked-upstream-cache'
-
-/**
- * #26948: `git switch -c x origin/develop` makes a branch track the default
- * branch, whose PRs (e.g. develop -> release) belong to that branch, not this one.
- * A same-named branch on a fork outside the PR candidates is a real head, so it stays.
- */
-export async function isTrackedUpstreamDefaultBranch(input: {
-  upstreamBranch: TrackedUpstreamBranch
-  upstreamHeadRepo: OwnerRepo
-  candidates: OwnerRepo[]
-  repoPath: string
-  connectionId?: string | null
-  localGitOptions: HostedReviewLocalGitOptions
-}): Promise<boolean> {
-  const upstreamRepoKey = githubRepoIdentityKey(input.upstreamHeadRepo)
-  if (!input.candidates.some((candidate) => githubRepoIdentityKey(candidate) === upstreamRepoKey)) {
-    return false
-  }
-  const { remoteName } = input.upstreamBranch
-  // Why: a fork's `upstream` can default to a different branch than `origin`; use origin's only when the remote never recorded its HEAD.
-  const remoteDefaultBranchName =
-    remoteName === 'origin'
-      ? null
-      : await getRemoteHeadBranchName(
-          input.repoPath,
-          remoteName,
-          input.connectionId,
-          input.localGitOptions
-        )
-  const defaultBranchName =
-    remoteDefaultBranchName ??
-    (await getRepoDefaultBranchName(input.repoPath, input.connectionId, input.localGitOptions))
-  return defaultBranchName === input.upstreamBranch.branchName
-}