1 files changed, 14 insertions, 3 deletions
diff --git a/internal/engine/tick.go b/internal/engine/tick.go
index aa10a73..68dd54f 100644
--- a/internal/engine/tick.go
+++ b/internal/engine/tick.go
@@ -26,6 +26,7 @@ type Engine struct {
 	ticker      *time.Ticker
 	running     bool
 	stopCh      chan struct{}
+	loopDone    chan struct{}
 }
 
 func New() *Engine {
@@ -67,8 +68,11 @@ func (e *Engine) Start(tickLengthMs int) {
 	// Capture this run's resources. A quick Stop/Start must not make the
 	// old goroutine consume the replacement ticker or stop channel.
 	ticker, stopCh := e.ticker, e.stopCh
+	loopDone := make(chan struct{})
+	e.loopDone = loopDone
 
 	go func() {
+		defer close(loopDone)
 		for {
 			select {
 			case <-ticker.C:
@@ -100,10 +104,17 @@ func (e *Engine) Stop() {
 // exited. Use this for graceful shutdown, where the final tick must finish
 // before owned state is saved.
 func (e *Engine) StopAndWait() {
+	e.mu.Lock()
+	loopDone := e.loopDone
+	e.mu.Unlock()
 	e.Stop()
-	// Join any in-flight tick: processTick holds tickMu for its whole body,
-	// so acquiring the mutex blocks until the running callback completes;
-	// holding it until return keeps that guarantee for the caller.
+	// Wait for the ticker loop goroutine to exit. This is the real save
+	// barrier: the loop may already have received from the ticker and be
+	// about to call processTick, which locking tickMu alone would not catch.
+	if loopDone != nil {
+		<-loopDone
+	}
+	// Also join a direct Tick() that is mid-callback on another goroutine.
 	e.tickMu.Lock()
 	defer e.tickMu.Unlock()
 }