fix: libgit2 branch checkout to avoid half-switched HEAD

src-tauri/src/git/mod.rs

@@ -8,8 +8,8 @@ use tauri::State;
 
 use crate::shared::process_core::tokio_command;
 use crate::git_utils::{
-    checkout_branch, commit_to_entry, diff_patch_to_string, diff_stats_for_path,
-    image_mime_type, list_git_roots as scan_git_roots, parse_github_repo, resolve_git_root,
+    checkout_branch, commit_to_entry, diff_patch_to_string, diff_stats_for_path, image_mime_type,
+    list_git_roots as scan_git_roots, parse_github_repo, resolve_git_root,
 };
 use crate::state::AppState;
 use crate::types::{
@@ -1497,6 +1497,7 @@ pub(crate) async fn create_git_branch(
 mod tests {
     use super::*;
     use std::fs;
+    use std::path::Path;
 
     fn create_temp_repo() -> (PathBuf, Repository) {
         let root = std::env::temp_dir().join(format!(

src-tauri/src/git_utils.rs

@@ -38,10 +38,12 @@ pub(crate) fn commit_to_entry(commit: git2::Commit) -> GitLogEntry {
 
 pub(crate) fn checkout_branch(repo: &Repository, name: &str) -> Result<(), git2::Error> {
     let refname = format!("refs/heads/{name}");
-    repo.set_head(&refname)?;
+    let target = repo.revparse_single(&refname)?;
+
     let mut options = git2::build::CheckoutBuilder::new();
     options.safe();
-    repo.checkout_head(Some(&mut options))?;
+    repo.checkout_tree(&target, Some(&mut options))?;
+    repo.set_head(&refname)?;
     Ok(())
 }
 
@@ -90,7 +92,10 @@ pub(crate) fn diff_patch_to_string(patch: &mut git2::Patch) -> Result<String, gi
 
 #[cfg(test)]
 mod tests {
-    use super::image_mime_type;
+    use super::{checkout_branch, image_mime_type};
+    use git2::Repository;
+    use std::fs;
+    use std::path::Path;
 
     #[test]
     fn image_mime_type_detects_known_extensions() {
@@ -101,6 +106,29 @@ mod tests {
         assert_eq!(image_mime_type("readme.txt"), None);
     }
 
+    #[test]
+    fn checkout_branch_missing_does_not_change_head() {
+        let root = std::env::temp_dir().join(format!(
+            "codex-monitor-git-utils-test-{}",
+            uuid::Uuid::new_v4()
+        ));
+        fs::create_dir_all(&root).expect("create temp repo root");
+        let repo = Repository::init(&root).expect("init repo");
+
+        fs::write(root.join("base.txt"), "base\n").expect("write file");
+        let mut index = repo.index().expect("index");
+        index.add_path(Path::new("base.txt")).expect("add path");
+        let tree_id = index.write_tree().expect("write tree");
+        let tree = repo.find_tree(tree_id).expect("find tree");
+        let sig = git2::Signature::now("Test", "test@example.com").expect("signature");
+        repo.commit(Some("HEAD"), &sig, &sig, "init", &tree, &[])
+            .expect("commit");
+
+        let before = repo.head().expect("head").name().unwrap_or("").to_string();
+        assert!(checkout_branch(&repo, "does-not-exist").is_err());
+        let after = repo.head().expect("head after").name().unwrap_or("").to_string();
+        assert_eq!(after, before);
+    }
 }
 
 pub(crate) fn parse_github_repo(remote_url: &str) -> Option<String> {