fix for inflated diff counts when base branch has been updated since stack init

internal/tui/stackview/data.go

@@ -45,15 +45,13 @@ func LoadBranchNodes(cfg *config.Config, s *stack.Stack, currentBranch string) [
 			node.IsLinear = isAncestor
 		}
 
-		// For merged branches, use the merge-base (fork point) as the diff
-		// anchor since the base branch has moved past the merge point and
-		// a two-dot diff would show nothing after a squash merge.
-		isMerged := b.IsMerged()
+		// Use the merge-base (fork point) as the diff anchor so that we
+		// only show changes introduced on this branch. Without this, a
+		// diverged base (e.g. local main ahead of the branch's fork point)
+		// would inflate the diff with unrelated files.
 		diffBase := baseBranch
-		if isMerged {
-			if mb, err := git.MergeBase(baseBranch, b.Branch); err == nil {
-				diffBase = mb
-			}
+		if mb, err := git.MergeBase(baseBranch, b.Branch); err == nil {
+			diffBase = mb
 		}
 
 		// Fetch commit range

internal/tui/stackview/data_test.go

@@ -0,0 +1,102 @@
+package stackview
+
+import (
+	"testing"
+
+	"github.com/github/gh-stack/internal/config"
+	"github.com/github/gh-stack/internal/git"
+	ghapi "github.com/github/gh-stack/internal/github"
+	"github.com/github/gh-stack/internal/stack"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLoadBranchNodes_UsesMergeBaseForDivergedBranch(t *testing.T) {
+	// Scenario: local main has diverged from the branch's history.
+	// Without merge-base, diff would be computed against main directly,
+	// inflating the file count with unrelated changes.
+	s := &stack.Stack{
+		Trunk:    stack.BranchRef{Branch: "main"},
+		Branches: []stack.BranchRef{{Branch: "feature"}},
+	}
+
+	var diffBase string
+	restore := git.SetOps(&git.MockOps{
+		IsAncestorFn: func(ancestor, descendant string) (bool, error) {
+			// main is NOT an ancestor of feature (diverged)
+			return false, nil
+		},
+		MergeBaseFn: func(a, b string) (string, error) {
+			return "abc123", nil
+		},
+		LogRangeFn: func(base, head string) ([]git.CommitInfo, error) {
+			return []git.CommitInfo{{SHA: "def456", Subject: "only commit"}}, nil
+		},
+		DiffStatFilesFn: func(base, head string) ([]git.FileDiffStat, error) {
+			diffBase = base
+			return []git.FileDiffStat{
+				{Path: "file1.go", Additions: 5, Deletions: 2},
+				{Path: "file2.go", Additions: 3, Deletions: 1},
+			}, nil
+		},
+	})
+	defer restore()
+
+	cfg, outW, errW := config.NewTestConfig()
+	defer outW.Close()
+	defer errW.Close()
+	// Ensure no real GitHub API calls
+	cfg.GitHubClientOverride = &ghapi.MockClient{}
+
+	nodes := LoadBranchNodes(cfg, s, "feature")
+
+	require.Len(t, nodes, 1)
+	// Diff must be computed from merge-base, not from "main" directly.
+	assert.Equal(t, "abc123", diffBase, "diff should use merge-base as base, not the branch name")
+	assert.Len(t, nodes[0].FilesChanged, 2)
+	assert.Equal(t, 8, nodes[0].Additions)
+	assert.Equal(t, 3, nodes[0].Deletions)
+	assert.False(t, nodes[0].IsLinear)
+}
+
+func TestLoadBranchNodes_LinearBranchStillUsesMergeBase(t *testing.T) {
+	// When base IS an ancestor (linear history), merge-base returns the
+	// base tip, so behavior is unchanged.
+	s := &stack.Stack{
+		Trunk:    stack.BranchRef{Branch: "main"},
+		Branches: []stack.BranchRef{{Branch: "feature"}},
+	}
+
+	var diffBase string
+	restore := git.SetOps(&git.MockOps{
+		IsAncestorFn: func(ancestor, descendant string) (bool, error) {
+			return true, nil
+		},
+		MergeBaseFn: func(a, b string) (string, error) {
+			// For linear history, merge-base returns the base tip
+			return "main-tip-sha", nil
+		},
+		LogRangeFn: func(base, head string) ([]git.CommitInfo, error) {
+			return nil, nil
+		},
+		DiffStatFilesFn: func(base, head string) ([]git.FileDiffStat, error) {
+			diffBase = base
+			return []git.FileDiffStat{
+				{Path: "only.go", Additions: 1, Deletions: 0},
+			}, nil
+		},
+	})
+	defer restore()
+
+	cfg, outW, errW := config.NewTestConfig()
+	defer outW.Close()
+	defer errW.Close()
+	cfg.GitHubClientOverride = &ghapi.MockClient{}
+
+	nodes := LoadBranchNodes(cfg, s, "other")
+
+	require.Len(t, nodes, 1)
+	assert.Equal(t, "main-tip-sha", diffBase)
+	assert.Len(t, nodes[0].FilesChanged, 1)
+	assert.True(t, nodes[0].IsLinear)
+}