fix(show): default to the top-of-status commit, not raw HEAD

src/core/graph.rs

@@ -156,21 +156,22 @@ fn display_path(repo_path: &str, cwd_prefix: &str) -> String {
 
 // ── Section building ────────────────────────────────────────────────────
 
-/// Group commits into sections: working changes, feature branches, loose
-/// commits, and the upstream marker. Commits are assigned to a branch when
-/// they follow a branch tip in topological order.
-fn build_sections(info: RepoInfo) -> Vec<Section> {
+/// Assign each in-range commit to the feature branch that owns it by walking
+/// parent links from every branch tip. Commits absent from the returned map
+/// are "loose": they sit on the integration line and belong to no feature
+/// branch.
+fn assign_commits_to_branches(info: &RepoInfo) -> HashMap<git2::Oid, String> {
     // Build a set of branch tip OIDs for quick lookup.
     let branch_tip_set: HashSet<git2::Oid> = info.branches.iter().map(|b| b.tip_oid).collect();
 
     // Group branches by tip OID to handle co-located branches (multiple
     // branch names pointing to the same commit).
-    let mut tip_to_names: HashMap<git2::Oid, Vec<(String, Option<RemoteStatus>)>> = HashMap::new();
+    let mut tip_to_names: HashMap<git2::Oid, Vec<String>> = HashMap::new();
     for b in &info.branches {
         tip_to_names
             .entry(b.tip_oid)
             .or_default()
-            .push((b.name.clone(), b.remote.clone()));
+            .push(b.name.clone());
     }
 
     // Build a parent lookup from the commit list so we can walk ancestry chains.
@@ -188,7 +189,7 @@ fn build_sections(info: RepoInfo) -> Vec<Section> {
         if !seen_tips.insert(b.tip_oid) {
             continue; // Already processed commits for this tip
         }
-        let canonical_name = tip_to_names[&b.tip_oid][0].0.clone();
+        let canonical_name = tip_to_names[&b.tip_oid][0].clone();
         let mut current = Some(b.tip_oid);
         let mut is_tip = true;
         while let Some(oid) = current {
@@ -205,6 +206,36 @@ fn build_sections(info: RepoInfo) -> Vec<Section> {
         }
     }
 
+    commit_to_branch
+}
+
+/// OID of the commit shown at the top of the status graph: the first loose
+/// commit (on the integration line, belonging to no feature branch) in
+/// topological order. Returns None when every in-range commit belongs to a
+/// feature branch, or there are no commits.
+pub fn top_loose_commit(info: &RepoInfo) -> Option<git2::Oid> {
+    let commit_to_branch = assign_commits_to_branches(info);
+    info.commits
+        .iter()
+        .find(|c| !commit_to_branch.contains_key(&c.oid))
+        .map(|c| c.oid)
+}
+
+/// Group commits into sections: working changes, feature branches, loose
+/// commits, and the upstream marker.
+fn build_sections(info: RepoInfo) -> Vec<Section> {
+    let commit_to_branch = assign_commits_to_branches(&info);
+
+    // Group branches by tip OID to handle co-located branches (multiple
+    // branch names pointing to the same commit).
+    let mut tip_to_names: HashMap<git2::Oid, Vec<(String, Option<RemoteStatus>)>> = HashMap::new();
+    for b in &info.branches {
+        tip_to_names
+            .entry(b.tip_oid)
+            .or_default()
+            .push((b.name.clone(), b.remote.clone()));
+    }
+
     // Map canonical name → all names for that branch group.
     // Reverse so the newest (alphabetically last) branch appears on top.
     let mut canonical_to_names: HashMap<String, Vec<(String, Option<RemoteStatus>)>> =

src/show.rs

@@ -2,15 +2,22 @@ use anyhow::Result;
 
 use crate::core::repo::{self, Target};
 use crate::git;
+use crate::status;
 
 /// Show the diff and metadata for a commit (like `git show`), using short IDs.
 ///
-/// With no target, shows the last commit on the current branch (like `git show`).
+/// With no target, shows the commit at the top of `loom status` — the tip of
+/// the integration line, skipping merge commits and hidden branches.
 pub fn run(target: Option<String>) -> Result<()> {
     let repo = repo::open_repo()?;
 
     let git_ref = match target {
-        None => "HEAD".to_string(),
+        // Fall back to HEAD outside an integration branch (e.g. plain repo)
+        // or when the integration line has no commits of its own.
+        None => match status::top_commit(&repo) {
+            Ok(Some(oid)) => oid.to_string(),
+            _ => "HEAD".to_string(),
+        },
         Some(target) => {
             let resolved = repo::resolve_arg(
                 &repo,

src/status.rs

@@ -47,6 +47,21 @@ pub fn run(
     Ok(())
 }
 
+/// OID of the commit shown at the top of `loom status`: the tip of the
+/// integration line, skipping merge commits and hidden branches. Returns None
+/// when the integration branch has no commits of its own above the merge-base.
+pub fn top_commit(repo: &git2::Repository) -> Result<Option<git2::Oid>> {
+    let mut info = repo::gather_repo_info(repo, false, 0)?;
+
+    let pattern =
+        repo::hide_branch_pattern(repo).unwrap_or_else(|| repo::DEFAULT_HIDE_PATTERN.to_string());
+    if !pattern.is_empty() {
+        hide_branches(&mut info, &pattern);
+    }
+
+    Ok(graph::top_loose_commit(&info))
+}
+
 /// Resolve a list of user-supplied IDs to a set of commit OIDs whose files
 /// should be shown. Supports git hashes and loom commit short IDs.
 /// Unknown IDs are silently skipped.

src/status_test.rs

@@ -126,6 +126,47 @@ fn multiple_hidden_branches() {
     assert!(info.commits.is_empty());
 }
 
+// ── top_commit tests ────────────────────────────────────────────────────────
+
+#[test]
+fn top_commit_returns_tip_loose_commit() {
+    let test_repo = TestRepo::new_with_remote();
+
+    test_repo.commit_empty("C1");
+    test_repo.commit_empty("C2");
+    let c2 = test_repo.head_oid();
+
+    let top = super::top_commit(&test_repo.repo).unwrap();
+    assert_eq!(top, Some(c2));
+}
+
+#[test]
+fn top_commit_skips_merge_of_hidden_branch() {
+    // Reproduces the bug: HEAD is a merge of a hidden `local-` branch. The top
+    // of status is the integration commit below the merge, not the merge itself.
+    let test_repo = TestRepo::new_with_remote();
+
+    test_repo.commit_empty("Base");
+    let base = test_repo.head_oid();
+
+    // A local-only branch that diverges from the base with its own commit.
+    test_repo.create_branch_at_commit("local-only", base);
+    test_repo.switch_branch("local-only");
+    test_repo.commit_empty("L1");
+
+    // Meanwhile the integration line advances with a real commit.
+    test_repo.switch_branch("integration");
+    test_repo.commit_empty("C1");
+    let c1 = test_repo.head_oid();
+
+    // Merge local-only back into integration, leaving a merge commit at the tip.
+    test_repo.merge_no_ff("local-only");
+    assert!(test_repo.head_commit().parent_count() > 1, "tip is a merge");
+
+    let top = super::top_commit(&test_repo.repo).unwrap();
+    assert_eq!(top, Some(c1), "should skip the merge and pick C1");
+}
+
 // ── resolve_commit_filter tests ─────────────────────────────────────────────
 
 #[test]