Fix stale merged-PR rebase boundaries

Cache merged PR heads independently of local branch snapshots and compare fallback cutoffs by ancestry. Backfill existing stacks and preserve worktree recovery boundaries.

Author: skarim

cmd/checkout.go

@@ -685,13 +685,8 @@ func importRemoteStack(
 	branchRefs := make([]stack.BranchRef, len(prs))
 	for i, pr := range prs {
 		branchRefs[i] = stack.BranchRef{
-			Branch: pr.HeadRefName,
-			PullRequest: &stack.PullRequestRef{
-				Number: pr.Number,
-				ID:     pr.ID,
-				URL:    pr.URL,
-				Merged: pr.Merged,
-			},
+			Branch:      pr.HeadRefName,
+			PullRequest: pullRequestRef(pr, nil),
 		}
 	}
 
@@ -727,12 +722,7 @@ func syncRemotePRState(s *stack.Stack, prs []*github.PullRequest) {
 		if !ok {
 			continue
 		}
-		s.Branches[i].PullRequest = &stack.PullRequestRef{
-			Number: pr.Number,
-			ID:     pr.ID,
-			URL:    pr.URL,
-			Merged: pr.Merged,
-		}
+		s.Branches[i].PullRequest = pullRequestRef(pr, s.Branches[i].PullRequest)
 		s.Branches[i].Queued = pr.IsQueued()
 	}
 }

cmd/checkout_test.go

@@ -1151,7 +1151,7 @@ func TestCheckout_NumericTarget_MergedBranchDeletedFromRemote(t *testing.T) {
 		},
 		FindPRByNumberFn: func(number int) (*github.PullRequest, error) {
 			prs := map[int]*github.PullRequest{
-				10: {ID: "PR_10", Number: 10, HeadRefName: "feat-1", BaseRefName: "main", Merged: true, State: "MERGED", URL: "https://github.com/o/r/pull/10"},
+				10: {ID: "PR_10", Number: 10, HeadRefName: "feat-1", HeadRefOID: "merged-head", BaseRefName: "main", Merged: true, State: "MERGED", URL: "https://github.com/o/r/pull/10"},
 				11: {ID: "PR_11", Number: 11, HeadRefName: "feat-2", BaseRefName: "feat-1", State: "OPEN", URL: "https://github.com/o/r/pull/11"},
 			}
 			return prs[number], nil
@@ -1165,6 +1165,24 @@ func TestCheckout_NumericTarget_MergedBranchDeletedFromRemote(t *testing.T) {
 	assert.Equal(t, "feat-2", checkedOut)
 	assert.Contains(t, output, "Skipping merged branch feat-1")
 	assert.Contains(t, output, "Imported stack with 2 branches")
+	sf, err := stack.Load(gitDir)
+	require.NoError(t, err)
+	assert.Equal(t, "merged-head", sf.Stacks[0].Branches[0].PullRequest.MergedHead)
+	assert.Empty(t, sf.Stacks[0].Branches[0].Head)
+}
+
+func TestSyncRemotePRState_MergedHead(t *testing.T) {
+	s := &stack.Stack{Branches: []stack.BranchRef{{
+		Branch: "parent", Head: "local",
+		PullRequest: &stack.PullRequestRef{Number: 10, Merged: true, MergedHead: "cached"},
+	}}}
+	syncRemotePRState(s, []*github.PullRequest{{Number: 10, HeadRefName: "parent", Merged: true}})
+	assert.Equal(t, "cached", s.Branches[0].PullRequest.MergedHead)
+	assert.Equal(t, "local", s.Branches[0].Head)
+
+	syncRemotePRState(s, []*github.PullRequest{{Number: 11, HeadRefName: "parent", State: "OPEN"}})
+	assert.Empty(t, s.Branches[0].PullRequest.MergedHead)
+	assert.False(t, s.Branches[0].IsMerged())
 }
 
 func TestCheckout_NumericTarget_AllPRsMerged(t *testing.T) {

cmd/rebase.go

@@ -250,10 +250,8 @@ func runRebase(cfg *config.Config, opts *rebaseOptions) error {
 	if startIdx > 0 {
 		prev := s.Branches[startIdx-1]
 		if prev.IsMerged() {
-			if sha, ok := originalRefs[prev.Branch]; ok {
-				needsOnto = true
-				ontoOldBase = sha
-			}
+			needsOnto = true
+			ontoOldBase = originalRefs[prev.Branch]
 		}
 	}