fix: Undo button now properly truncates move history

- Added UNDO_TO_MOVE action that both navigates AND truncates history

- Added undoToMove() function to OpeningTrainingContext

- Updated handleUndoDeviation to use undoToMove instead of navigate

Previously, the Undo button only navigated back but didn't remove

the deviation moves, so the session remained in off-book state.

src/components/OpeningTrainer/OpeningTrainer.tsx

@@ -52,6 +52,7 @@ export default function OpeningTrainer({
     currentFeedback,
     initializeSession,
     makeMove,
+    undoToMove,
     navigateToMove,
   } = useOpeningTraining();
 
@@ -225,12 +226,12 @@ export default function OpeningTrainer({
   const handleUndoDeviation = () => {
     if (!session || session.deviationMoveIndex === null) return;
 
-    // Navigate back to the move before deviation
-    navigateToMove(session.deviationMoveIndex - 1);
-    setShowDeviationDialog(false);
+    // Undo to the move before deviation (this truncates move history)
+    const targetIndex = session.deviationMoveIndex - 1;
+    undoToMove(targetIndex);
 
-    // After a brief delay, make another legal move to continue in theory
-    // This allows the player to try again
+    // Close the dialog
+    setShowDeviationDialog(false);
   };
 
   const handleStartGameFromPosition = () => {

src/contexts/OpeningTrainingContext.tsx

@@ -38,6 +38,7 @@ interface OpeningTrainingContextType {
   // Actions
   initializeSession: (opening: OpeningMetadata, forceNew?: boolean) => Promise<void>;
   makeMove: (san: string) => Promise<void>;
+  undoToMove: (index: number) => void;
   navigateToMove: (index: number) => void;
   resetSession: () => void;
 }
@@ -294,6 +295,17 @@ export function OpeningTrainingProvider({
     }
   };
 
+  const undoToMove = (index: number) => {
+    if (!session) return;
+
+    console.log('[OpeningTraining] Undoing to move index:', index);
+
+    dispatch({ type: 'UNDO_TO_MOVE', index });
+
+    // Clear feedback when undoing
+    setCurrentFeedback(null);
+  };
+
   const navigateToMove = (index: number) => {
     if (!session) return;
 
@@ -355,6 +367,7 @@ export function OpeningTrainingProvider({
     currentFeedback,
     initializeSession,
     makeMove,
+    undoToMove,
     navigateToMove,
     resetSession,
   };

src/lib/openingTrainer/sessionReducer.ts

@@ -93,6 +93,7 @@ export type SessionAction =
   | { type: 'NAVIGATE_PREVIOUS' }
   | { type: 'NAVIGATE_NEXT' }
   | { type: 'NAVIGATE_TO_CURRENT' } // Jump back to end of history
+  | { type: 'UNDO_TO_MOVE'; index: number } // Navigate and truncate history
 
   // Theory tracking
   | { type: 'DEVIATION_DETECTED'; moveIndex: number }
@@ -348,6 +349,47 @@ export function sessionReducer(
       });
     }
 
+    case 'UNDO_TO_MOVE': {
+      const { index } = action;
+
+      // Clamp index to valid range (-1 means start position, truncate all moves)
+      const clampedIndex = Math.max(-1, Math.min(index, state.moveHistory.length - 1));
+
+      // Truncate move history to this point
+      const newHistory = clampedIndex < 0 ? [] : state.moveHistory.slice(0, clampedIndex + 1);
+
+      // Build FEN at target index
+      const newFEN = buildFenAtIndex(newHistory, newHistory.length);
+
+      // Reset deviation if we're now back in theory
+      const isNowInTheory = isInTheory(state.opening, newHistory);
+      const newDeviationIndex = isNowInTheory ? null : state.deviationMoveIndex;
+
+      // Determine phase
+      let phase: SessionPhase;
+      if (newDeviationIndex !== null) {
+        phase = 'off_book';
+      } else if (isEndOfRepertoire(state.opening, newHistory.length)) {
+        phase = 'end_of_repertoire';
+      } else if (isOpponentTurn(state.opening, newHistory)) {
+        phase = 'opponent_turn';
+      } else {
+        phase = 'user_turn';
+      }
+
+      return {
+        ...state,
+        moveHistory: newHistory,
+        currentMoveIndex: newHistory.length,
+        currentFEN: newFEN,
+        deviationMoveIndex: newDeviationIndex,
+        isInTheory: isNowInTheory,
+        phase,
+        pendingOpponentMove: null,
+        lastUpdatedAt: Date.now(),
+      };
+    }
+
     case 'NAVIGATE_TO_CURRENT': {
       // Jump to the end of history (current position)
       const atEnd = state.currentMoveIndex === state.moveHistory.length;