Add test for auto-forwarding a branch checked out in another worktree

When the linked worktree's branch is changed externally — by another
shell, by another tool, or by git running outside lazygit — lazygit's
worktrees model goes stale. The next post-fetch auto-forward then
doesn't realise the branch is now checked out elsewhere, and advances
its ref behind the worktree's back. The worktree's HEAD then resolves
to a commit its index/working tree haven't been updated to, and the
user sees that diff as the inverse of what was fetched — files
appearing as pending changes that they didn't make.

The test sets up a linked worktree initially on a side branch, then
externally checks out master in it before pressing fetch. Two
EXPECTED/ACTUAL pairs capture the symptoms: the branches view shows
master as `✓` rather than `↓1`, and switching to the linked worktree
shows master's would-be incoming file as a pending deletion against
HEAD.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/integration/tests/sync/fetch_and_auto_forward_branches_worktree_added_after_startup.go

@@ -0,0 +1,68 @@
+package sync
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var FetchAndAutoForwardBranchesWorktreeAddedAfterStartup = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Auto-forward skips a main branch that was externally checked out in a linked worktree after lazygit started",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(config *config.AppConfig) {
+		config.GetUserConfig().Git.AutoForwardBranches = "onlyMainBranches"
+		config.GetUserConfig().Git.LocalBranchSortOrder = "alphabetical"
+	},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateNCommits(3)
+		shell.NewBranch("feature")
+		shell.NewBranch("wt-branch")
+		shell.CloneIntoRemote("origin")
+		shell.SetBranchUpstream("master", "origin/master")
+		shell.SetBranchUpstream("feature", "origin/feature")
+		shell.Checkout("master")
+		shell.HardReset("HEAD^")
+		shell.Checkout("feature")
+		shell.AddWorktreeCheckout("wt-branch", "../linked-worktree")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Branches().
+			Lines(
+				Contains("feature").IsSelected(),
+				Contains("master ↓1").DoesNotContain("↑"),
+				Contains("wt-branch (worktree linked-worktree)"),
+			)
+
+		// Switch the linked worktree to master externally.
+		t.Shell().RunCommand([]string{"git", "-C", "../linked-worktree", "checkout", "master"})
+
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Files.Fetch)
+
+		t.Views().Branches().
+			Lines(
+				Contains("feature").IsSelected(),
+				/* EXPECTED:
+				Contains("master (worktree linked-worktree) ↓1"),
+				Contains("wt-branch").DoesNotContain("worktree"),
+				ACTUAL: */
+				Contains("master ✓"),
+				Contains("wt-branch (worktree linked-worktree)"),
+			)
+
+		t.Views().Worktrees().
+			Focus().
+			NavigateToLine(Contains("linked-worktree")).
+			PressPrimaryAction()
+
+		t.Views().Files().
+			Focus().
+			/* EXPECTED:
+			IsEmpty()
+			ACTUAL: */
+			Lines(
+				Equals("D  file03.txt"),
+			)
+	},
+})

pkg/integration/tests/test_list.go

@@ -428,6 +428,7 @@ var tests = []*components.IntegrationTest{
 	sync.FetchAndAutoForwardBranchesAllBranchesCheckedOutInOtherWorktree,
 	sync.FetchAndAutoForwardBranchesNone,
 	sync.FetchAndAutoForwardBranchesOnlyMainBranches,
+	sync.FetchAndAutoForwardBranchesWorktreeAddedAfterStartup,
 	sync.FetchPrune,
 	sync.FetchWhenSortedByDate,
 	sync.ForcePush,