fix: skip auto-commit when committed reading is shorter than 2 kana

engine/crates/lex-session/src/auto_commit.rs

@@ -12,7 +12,7 @@ impl InputSession {
         }
         // Extract data from comp() in a block so the borrow is dropped before
         // we access self.history_records.
-        let (committed_reading, committed_surface, seg_pairs, commit_count) = {
+        let (committed_reading, committed_surface, committed_reading_len, seg_pairs, commit_count) = {
             let c = self.comp();
             if c.stability.count < 3 {
                 return None;
@@ -41,6 +41,17 @@ impl InputSession {
             let segments: Vec<&ConvertedSegment> = best_path[0..commit_count].iter().collect();
             let committed_reading: String = segments.iter().map(|s| s.reading.as_str()).collect();
             let committed_surface: String = segments.iter().map(|s| s.surface.as_str()).collect();
+            let committed_reading_len = committed_reading.chars().count();
+
+            // Skip auto-commit if the committed reading is a single kana.
+            // Single-kana commits (e.g. "じ" from "じぇのさいど") destroy longer
+            // words that haven't been fully typed yet.  Only applies to kana
+            // readings — ASCII prefixes (e.g. URLs) are not subject to this guard.
+            if lex_core::unicode::is_hiragana_reading(&committed_reading)
+                && committed_reading_len < 2
+            {
+                return None;
+            }
 
             if !c.kana.starts_with(&committed_reading) {
                 return None;
@@ -60,6 +71,7 @@ impl InputSession {
             (
                 committed_reading,
                 committed_surface,
+                committed_reading_len,
                 seg_pairs,
                 commit_count,
             )
@@ -78,7 +90,7 @@ impl InputSession {
         // Safety: starts_with check above guarantees the byte offset is a valid
         // UTF-8 boundary, but we use char-based slicing for extra safety.
         let c = self.comp();
-        let skip_chars = committed_reading.chars().count();
+        let skip_chars = committed_reading_len;
         c.kana = c.kana.chars().skip(skip_chars).collect();
         c.stability.reset();
 

engine/crates/lex-session/src/tests/candidates.rs

@@ -240,3 +240,54 @@ fn test_predictive_mode_no_auto_commit() {
         );
     }
 }
+
+#[test]
+fn test_auto_commit_skips_single_kana_first_segment() {
+    use lex_core::candidates::generate_candidates;
+
+    let dict = make_test_dict();
+    let mut session = InputSession::new(dict.clone(), None, None);
+    session.set_defer_candidates(true);
+
+    fn complete_cycle(session: &mut InputSession, dict: &dyn Dictionary) -> Option<KeyResponse> {
+        let reading = session.comp().kana.clone();
+        if reading.is_empty() {
+            return None;
+        }
+        let cand = generate_candidates(dict, None, None, &reading, 20);
+        session.receive_candidates(&reading, cand.surfaces, cand.paths)
+    }
+
+    // Type a reading where Viterbi may produce a single-kana first segment.
+    // Even if stability threshold is met, auto-commit must NOT fire when the
+    // committed reading is shorter than 2 kana.
+    type_string(&mut session, "ji");
+    complete_cycle(&mut session, &*dict);
+    type_string(&mut session, "ji");
+    complete_cycle(&mut session, &*dict);
+    type_string(&mut session, "ji");
+    complete_cycle(&mut session, &*dict);
+    type_string(&mut session, "ji");
+    let r = complete_cycle(&mut session, &*dict);
+
+    // Verify auto-commit preconditions are met (stability >= 3),
+    // so this test is actually exercising the min-length guard.
+    assert!(
+        session.comp().stability.count >= 3,
+        "stability count should reach threshold; got {}",
+        session.comp().stability.count
+    );
+
+    // Even after many cycles, a single-kana first segment must not auto-commit.
+    if let Some(ref resp) = r {
+        assert!(
+            resp.commit.is_none(),
+            "auto-commit should not commit when the first segment would be a single kana"
+        );
+    }
+    assert_eq!(
+        session.comp().kana,
+        "じじじじ",
+        "composing kana should remain unchanged when auto-commit is skipped"
+    );
+}