Improve engine: tone placement, backtrack limit, grab default

- Fix tone placement for uâ, uê, uơ → tone on second vowel (e.g. tuấn, thuế, phương)
- Limit flexible backward scan to 3 chars to prevent cross-syllable modification
- Set grab = true as default config
- Add 9 new engine tests for tone placement and backtrack limit

engine/src/telex.rs

@@ -2,6 +2,12 @@ use crate::engine::EngineEvent;
 
 const VOWELS: &[char] = &['a', 'e', 'i', 'o', 'u', 'y', 'ă', 'â', 'ê', 'ô', 'ơ', 'ư'];
 
+/// Maximum number of characters to scan backward during flexible placement.
+/// Vietnamese vowel clusters are at most 3 characters; limiting the scan
+/// prevents modifying vowels in a different syllable (e.g. `dang d` + `a`
+/// should not change the `a` in `dang`).
+const MAX_FLEXIBLE_BACKTRACK: usize = 3;
+
 fn is_vowel(c: char) -> bool {
     VOWELS.contains(&c)
 }
@@ -152,11 +158,12 @@ impl TelexEngine {
                     if i > 0 && is_vowel(chars[i - 1]) {
                         let first = chars[i - 1];
                         let second = chars[i];
-                        // For oa, oe, uy → tone on second vowel (already at position i)
-                        // For others → tone on first vowel
+                        // For oa, oe, uâ, uê, uơ, uy, iê, yê → tone on second vowel
                         let tone_on_second = matches!(
                             (first, second),
-                            ('o', 'a') | ('o', 'e') | ('u', 'y') | ('i', 'ê') | ('y', 'ê')
+                            ('o', 'a') | ('o', 'e')
+                            | ('u', 'â') | ('u', 'ê') | ('u', 'ơ') | ('u', 'y')
+                            | ('i', 'ê') | ('y', 'ê')
                         );
                         if !tone_on_second {
                             // Apply tone to first vowel
@@ -212,13 +219,15 @@ impl TelexEngine {
             }
         }
 
-        // Flexible placement: if last char is not a vowel, scan backward
-        // for a matching vowel to form a double-vowel pair.
+        // Flexible placement: if last char is not a vowel, scan the last
+        // N chars for a matching vowel to form a double-vowel pair.
+        // Limited backtrack prevents modifying vowels in a different syllable.
         if matches!(ch, 'a' | 'e' | 'o') {
             if let Some(last_ch) = self.buffer.chars().last() {
                 if !is_vowel(last_ch) {
                     let chars: Vec<char> = self.buffer.chars().collect();
-                    for i in (0..chars.len()).rev() {
+                    let start = chars.len().saturating_sub(MAX_FLEXIBLE_BACKTRACK);
+                    for i in (start..chars.len()).rev() {
                         if chars[i] == ch {
                             let replacement = match ch {
                                 'a' => 'â',
@@ -255,12 +264,13 @@ impl TelexEngine {
             }
         }
 
-        // Flexible placement: if last char is not a vowel, scan backward
-        // for a vowel to apply the w modifier.
+        // Flexible placement: if last char is not a vowel, scan the last
+        // N chars for a vowel to apply the w modifier.
         if let Some(last_ch) = self.buffer.chars().last() {
             if !is_vowel(last_ch) {
                 let chars: Vec<char> = self.buffer.chars().collect();
-                for i in (0..chars.len()).rev() {
+                let start = chars.len().saturating_sub(MAX_FLEXIBLE_BACKTRACK);
+                for i in (start..chars.len()).rev() {
                     if is_vowel(chars[i]) {
                         if let Some(modified) = apply_w_to_vowel(chars[i]) {
                             self.buffer = chars[..i].iter().collect::<String>();

engine/src/tests.rs

@@ -279,10 +279,69 @@ mod tests {
     #[test]
     fn telex_uy_tone() {
         let mut e = Engine::new(InputMethod::Telex);
-        // Engine applies tone to first vowel in "uy": uý
+        // Engine applies tone to second vowel (y) in "uy": uý
         assert_eq!(get_display(&process_input(&mut e, "uys")), "uý");
     }
 
+    #[test]
+    fn telex_ua_tone_on_first_vowel() {
+        let mut e = Engine::new(InputMethod::Telex);
+        // "ua" → tone on first vowel (u): mùa → "ùa"
+        assert_eq!(get_display(&process_input(&mut e, "uaf")), "ùa");
+    }
+
+    #[test]
+    fn telex_uâ_tone_on_second_vowel() {
+        let mut e = Engine::new(InputMethod::Telex);
+        // "uâ" → tone on second vowel (â): tuấn
+        assert_eq!(get_display(&process_input(&mut e, "tuana")), "tuân");
+        assert_eq!(get_display(&process_input(&mut e, "tuanas")), "tuấn");
+    }
+
+    #[test]
+    fn telex_uê_tone_on_second_vowel() {
+        let mut e = Engine::new(InputMethod::Telex);
+        // "uê" → tone on second vowel (ê): thuế
+        assert_eq!(get_display(&process_input(&mut e, "thuee")), "thuê");
+        assert_eq!(get_display(&process_input(&mut e, "thuees")), "thuế");
+    }
+
+    // ================================================================
+    // Telex: Flexible backtrack limit
+    // ================================================================
+
+    #[test]
+    fn telex_flexible_backtrack_limit() {
+        let mut e = Engine::new(InputMethod::Telex);
+        // "dangd" + "a" should NOT modify the 'a' in "dang"
+        // (too far back, crosses a syllable boundary).
+        // The last 3 chars are "ngd" → no vowel → 'a' is appended normally.
+        assert_eq!(get_display(&process_input(&mut e, "dangda")), "dangda");
+    }
+
+    #[test]
+    fn telex_flexible_backtrack_still_works_near() {
+        let mut e = Engine::new(InputMethod::Telex);
+        // "tran" + "a" → last 3: "ran" → 'a' found at index 1 → "trân"
+        assert_eq!(get_display(&process_input(&mut e, "trana")), "trân");
+    }
+
+    #[test]
+    fn telex_flexible_backtrack_w_limit() {
+        let mut e = Engine::new(InputMethod::Telex);
+        // "dangd" + "w" should NOT modify 'a' in "dang".
+        // w becomes a pending modifier (no vowel found within backtrack)
+        // On flush, pending w is consumed without modifying anything.
+        assert_eq!(get_display(&process_input(&mut e, "dangdw")), "dangd");
+    }
+
+    #[test]
+    fn telex_flexible_backtrack_w_still_works_near() {
+        let mut e = Engine::new(InputMethod::Telex);
+        // "ngon" + "w" → last 3: "gon" → 'o' found at index 1 → "ngơn"
+        assert_eq!(get_display(&process_input(&mut e, "ngonw")), "ngơn");
+    }
+
     // ================================================================
     // Telex: Digraph dd
     // ================================================================
@@ -624,6 +683,20 @@ mod tests {
         assert_eq!(get_display(&process_input(&mut e, "1")), "1");
     }
 
+    #[test]
+    fn vni_flexible_backtrack_limit() {
+        let mut e = Engine::new(InputMethod::Vni);
+        // "dangd" + "6" should NOT modify 'a' in "dang"
+        assert_eq!(get_display(&process_input(&mut e, "dangd6")), "dangd6");
+    }
+
+    #[test]
+    fn vni_flexible_backtrack_still_works_near() {
+        let mut e = Engine::new(InputMethod::Vni);
+        // "tran" + "6" → "trân" (within backtrack limit)
+        assert_eq!(get_display(&process_input(&mut e, "tran6")), "trân");
+    }
+
     // ================================================================
     // VNI: Tones
     // ================================================================

engine/src/vni.rs

@@ -6,6 +6,8 @@ fn is_vowel(c: char) -> bool {
     VOWELS.contains(&c)
 }
 
+const MAX_FLEXIBLE_BACKTRACK: usize = 3;
+
 fn apply_tone_to_vowel(vowel: char, digit: char) -> Option<char> {
     // VNI: 1=sắc, 2=huyền, 3=hỏi, 4=ngã, 5=nặng
     let table: &[(char, char, char)] = &[
@@ -132,11 +134,12 @@ impl VniEngine {
             }
         }
 
-        // Flexible placement: last char not a vowel, scan backward
+        // Flexible placement: last char not a vowel, scan the last N chars
         if let Some(last_ch) = self.buffer.chars().last() {
             if !is_vowel(last_ch) {
                 let chars: Vec<char> = self.buffer.chars().collect();
-                for i in (0..chars.len()).rev() {
+                let start = chars.len().saturating_sub(MAX_FLEXIBLE_BACKTRACK);
+                for i in (start..chars.len()).rev() {
                     if is_vowel(chars[i]) {
                         // Try tone first (1-5)
                         if let Some(modified) = apply_tone_to_vowel(chars[i], digit) {

vietc.toml

@@ -3,7 +3,7 @@
 input_method = "telex"
 toggle_key = "space"
 start_enabled = true
-grab = false
+grab = true
 
 [auto_restore]
 enabled = true