skip stale merged PRs when syncing branches with reused names

When a branch name was previously used for a merged PR,
syncStackPRs would adopt the old merged PR for the new branch,
falsely showing it as merged in gh stack view. Now, branches
with no recorded PR only adopt open PRs from the API.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: skarim

cmd/utils.go

@@ -252,6 +252,13 @@ func syncStackPRs(cfg *config.Config, s *stack.Stack) {
 			continue
 		}
 
+		// If this branch has no recorded PR yet and the API returned a
+		// merged or closed PR, skip it — it's likely from a prior use of
+		// the same branch name and doesn't belong to this stack.
+		if b.PullRequest == nil && (pr.Merged || pr.State == "CLOSED") {
+			continue
+		}
+
 		b.PullRequest = &stack.PullRequestRef{
 			Number: pr.Number,
 			ID:     pr.ID,

cmd/utils_test.go

@@ -3,12 +3,14 @@ package cmd
 import (
 	"errors"
 	"fmt"
+	"io"
 	"strings"
 	"testing"
 
 	"github.com/AlecAivazis/survey/v2/terminal"
 	"github.com/github/gh-stack/internal/config"
 	"github.com/github/gh-stack/internal/git"
+	"github.com/github/gh-stack/internal/github"
 	"github.com/github/gh-stack/internal/stack"
 	"github.com/stretchr/testify/assert"
 )
@@ -271,3 +273,104 @@ func TestParsePRURL(t *testing.T) {
 		})
 	}
 }
+
+func TestSyncStackPRs_SkipsStalesMergedPRForReusedBranch(t *testing.T) {
+	s := &stack.Stack{
+		Trunk: stack.BranchRef{Branch: "main"},
+		Branches: []stack.BranchRef{
+			{Branch: "feat-a"}, // No PullRequest recorded yet
+		},
+	}
+
+	cfg, _, errR := config.NewTestConfig()
+	cfg.GitHubClientOverride = &github.MockClient{
+		FindAnyPRForBranchFn: func(branch string) (*github.PullRequest, error) {
+			// Simulate a previously merged PR with the same branch name.
+			return &github.PullRequest{
+				Number: 20,
+				ID:     "PR_20",
+				URL:    "https://github.com/o/r/pull/20",
+				State:  "MERGED",
+				Merged: true,
+			}, nil
+		},
+	}
+
+	syncStackPRs(cfg, s)
+
+	cfg.Err.Close()
+	_, _ = io.ReadAll(errR)
+
+	// The merged PR from a prior use of the branch name should NOT be adopted.
+	assert.Nil(t, s.Branches[0].PullRequest, "should not adopt a merged PR for a branch with no recorded PR")
+	assert.False(t, s.Branches[0].IsMerged(), "branch should not appear merged")
+}
+
+func TestSyncStackPRs_AdoptsOpenPRForNewBranch(t *testing.T) {
+	s := &stack.Stack{
+		Trunk: stack.BranchRef{Branch: "main"},
+		Branches: []stack.BranchRef{
+			{Branch: "feat-b"}, // No PullRequest recorded yet
+		},
+	}
+
+	cfg, _, errR := config.NewTestConfig()
+	cfg.GitHubClientOverride = &github.MockClient{
+		FindAnyPRForBranchFn: func(branch string) (*github.PullRequest, error) {
+			return &github.PullRequest{
+				Number: 42,
+				ID:     "PR_42",
+				URL:    "https://github.com/o/r/pull/42",
+				State:  "OPEN",
+				Merged: false,
+			}, nil
+		},
+	}
+
+	syncStackPRs(cfg, s)
+
+	cfg.Err.Close()
+	_, _ = io.ReadAll(errR)
+
+	// An open PR should be adopted normally.
+	assert.NotNil(t, s.Branches[0].PullRequest, "should adopt an open PR")
+	assert.Equal(t, 42, s.Branches[0].PullRequest.Number)
+	assert.False(t, s.Branches[0].IsMerged())
+}
+
+func TestSyncStackPRs_UpdatesExistingPRMergedStatus(t *testing.T) {
+	s := &stack.Stack{
+		Trunk: stack.BranchRef{Branch: "main"},
+		Branches: []stack.BranchRef{
+			{
+				Branch: "feat-c",
+				PullRequest: &stack.PullRequestRef{
+					Number: 55,
+					ID:     "PR_55",
+				},
+			},
+		},
+	}
+
+	cfg, _, errR := config.NewTestConfig()
+	cfg.GitHubClientOverride = &github.MockClient{
+		FindAnyPRForBranchFn: func(branch string) (*github.PullRequest, error) {
+			return &github.PullRequest{
+				Number: 55,
+				ID:     "PR_55",
+				URL:    "https://github.com/o/r/pull/55",
+				State:  "MERGED",
+				Merged: true,
+			}, nil
+		},
+	}
+
+	syncStackPRs(cfg, s)
+
+	cfg.Err.Close()
+	_, _ = io.ReadAll(errR)
+
+	// When a PR is already recorded, merged status should be synced.
+	assert.NotNil(t, s.Branches[0].PullRequest)
+	assert.True(t, s.Branches[0].IsMerged(), "should update merged status for already-recorded PR")
+}