diff --git a/lib/tests/test_git.rs b/lib/tests/test_git.rs
index fa6e117fcc..5133b7632a 100644
--- a/lib/tests/test_git.rs
+++ b/lib/tests/test_git.rs
@@ -312,6 +312,109 @@ fn test_import_refs_reimport_git_head_counts() {
     assert!(tx.mut_repo().view().heads().contains(&jj_id(&commit)));
 }
 
+#[test]
+fn test_import_refs_reimport_git_head_without_ref() {
+    // Simulate external `git checkout` in colocated repo, from anonymous branch.
+    let settings = testutils::user_settings();
+    let git_settings = GitSettings::default();
+    let test_repo = TestRepo::init(true);
+    let repo = &test_repo.repo;
+    let git_repo = repo.store().git_repo().unwrap();
+
+    // First, HEAD points to commit1.
+    let mut tx = repo.start_transaction(&settings, "test");
+    let commit1 = write_random_commit(tx.mut_repo(), &settings);
+    let commit2 = write_random_commit(tx.mut_repo(), &settings);
+    git_repo.set_head_detached(git_id(&commit1)).unwrap();
+
+    // Import HEAD.
+    git::import_refs(tx.mut_repo(), &git_repo, &git_settings).unwrap();
+    tx.mut_repo().rebase_descendants(&settings).unwrap();
+    assert!(tx.mut_repo().view().heads().contains(commit1.id()));
+    assert!(tx.mut_repo().view().heads().contains(commit2.id()));
+
+    // Move HEAD to commit2 (by e.g. `git checkout` command)
+    git_repo.set_head_detached(git_id(&commit2)).unwrap();
+
+    // Reimport HEAD, which abandons the old HEAD branch because jj thinks it
+    // would be rewritten by e.g. `git commit --amend` command.
+    git::import_refs(tx.mut_repo(), &git_repo, &git_settings).unwrap();
+    tx.mut_repo().rebase_descendants(&settings).unwrap();
+    assert!(!tx.mut_repo().view().heads().contains(commit1.id()));
+    assert!(tx.mut_repo().view().heads().contains(commit2.id()));
+}
+
+#[test]
+fn test_import_refs_reimport_git_head_with_moved_ref() {
+    // Simulate external history rewriting in colocated repo.
+    let settings = testutils::user_settings();
+    let git_settings = GitSettings::default();
+    let test_repo = TestRepo::init(true);
+    let repo = &test_repo.repo;
+    let git_repo = repo.store().git_repo().unwrap();
+
+    // First, both HEAD and main point to commit1.
+    let mut tx = repo.start_transaction(&settings, "test");
+    let commit1 = write_random_commit(tx.mut_repo(), &settings);
+    let commit2 = write_random_commit(tx.mut_repo(), &settings);
+    git_repo
+        .reference("refs/heads/main", git_id(&commit1), true, "test")
+        .unwrap();
+    git_repo.set_head_detached(git_id(&commit1)).unwrap();
+
+    // Import HEAD and main.
+    git::import_refs(tx.mut_repo(), &git_repo, &git_settings).unwrap();
+    tx.mut_repo().rebase_descendants(&settings).unwrap();
+    assert!(tx.mut_repo().view().heads().contains(commit1.id()));
+    assert!(tx.mut_repo().view().heads().contains(commit2.id()));
+
+    // Move both HEAD and main to commit2 (by e.g. `git commit --amend` command)
+    git_repo
+        .reference("refs/heads/main", git_id(&commit2), true, "test")
+        .unwrap();
+    git_repo.set_head_detached(git_id(&commit2)).unwrap();
+
+    // Reimport HEAD and main, which abandons the old HEAD/main branch.
+    git::import_refs(tx.mut_repo(), &git_repo, &git_settings).unwrap();
+    tx.mut_repo().rebase_descendants(&settings).unwrap();
+    assert!(!tx.mut_repo().view().heads().contains(commit1.id()));
+    assert!(tx.mut_repo().view().heads().contains(commit2.id()));
+}
+
+#[test]
+fn test_import_refs_reimport_git_head_with_fixed_ref() {
+    // Simulate external `git checkout` in colocated repo, from named branch.
+    let settings = testutils::user_settings();
+    let git_settings = GitSettings::default();
+    let test_repo = TestRepo::init(true);
+    let repo = &test_repo.repo;
+    let git_repo = repo.store().git_repo().unwrap();
+
+    // First, both HEAD and main point to commit1.
+    let mut tx = repo.start_transaction(&settings, "test");
+    let commit1 = write_random_commit(tx.mut_repo(), &settings);
+    let commit2 = write_random_commit(tx.mut_repo(), &settings);
+    git_repo
+        .reference("refs/heads/main", git_id(&commit1), true, "test")
+        .unwrap();
+    git_repo.set_head_detached(git_id(&commit1)).unwrap();
+
+    // Import HEAD and main.
+    git::import_refs(tx.mut_repo(), &git_repo, &git_settings).unwrap();
+    tx.mut_repo().rebase_descendants(&settings).unwrap();
+    assert!(tx.mut_repo().view().heads().contains(commit1.id()));
+    assert!(tx.mut_repo().view().heads().contains(commit2.id()));
+
+    // Move only HEAD to commit2 (by e.g. `git checkout` command)
+    git_repo.set_head_detached(git_id(&commit2)).unwrap();
+
+    // Reimport HEAD, which shouldn't abandon the old HEAD branch.
+    git::import_refs(tx.mut_repo(), &git_repo, &git_settings).unwrap();
+    tx.mut_repo().rebase_descendants(&settings).unwrap();
+    assert!(tx.mut_repo().view().heads().contains(commit1.id()));
+    assert!(tx.mut_repo().view().heads().contains(commit2.id()));
+}
+
 #[test]
 fn test_import_refs_reimport_all_from_root_removed() {
     // Test that if a chain of commits all the way from the root gets unreferenced,
diff --git a/tests/test_git_colocated.rs b/tests/test_git_colocated.rs
index b32edac55f..5e8e4dfb5b 100644
--- a/tests/test_git_colocated.rs
+++ b/tests/test_git_colocated.rs
@@ -267,6 +267,44 @@ fn test_git_colocated_fetch_deleted_branch() {
     "###);
 }
 
+#[test]
+fn test_git_colocated_external_checkout() {
+    let test_env = TestEnvironment::default();
+    let repo_path = test_env.env_root().join("repo");
+    let git_repo = git2::Repository::init(&repo_path).unwrap();
+    test_env.jj_cmd_success(&repo_path, &["init", "--git-repo=."]);
+    test_env.jj_cmd_success(&repo_path, &["ci", "-m=A"]);
+    test_env.jj_cmd_success(&repo_path, &["new", "-m=B", "root"]);
+    test_env.jj_cmd_success(&repo_path, &["new"]);
+
+    // Checked out anonymous branch
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  53637cd508ff02427dd78eca98f5b2450a6370ce
+    ◉  66f4d1806ae41bd604f69155dece64062a0056cf
+    │ ◉  a86754f975f953fa25da4265764adc0c62e9ce6b master
+    ├─╯
+    ◉  0000000000000000000000000000000000000000
+    "###);
+
+    // Check out another branch by external command
+    git_repo
+        .set_head_detached(
+            git_repo
+                .find_reference("refs/heads/master")
+                .unwrap()
+                .target()
+                .unwrap(),
+        )
+        .unwrap();
+
+    // The old HEAD branch gets abandoned because jj thinks it has been rewritten.
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  0521ce3b8c4e29aab79f3c750e2845dcbc4c3874
+    ◉  a86754f975f953fa25da4265764adc0c62e9ce6b master
+    ◉  0000000000000000000000000000000000000000
+    "###);
+}
+
 #[test]
 fn test_git_colocated_squash_undo() {
     let test_env = TestEnvironment::default();