Fix data race

src/core.go

@@ -460,6 +460,11 @@ func Run(opts *Options) (int, error) {
 						}
 						if val.withNth != nil {
 							newTransformer := val.withNth.fn
+							// Cancel any in-flight scan and block the terminal from reading
+							// items before mutating them in-place. Snapshot shares middle
+							// chunk pointers, so the matcher and terminal can race with us.
+							matcher.CancelScan()
+							terminal.PauseRendering()
 							// Reset cross-line ANSI state before re-processing all items
 							lineAnsiState = nil
 							prevLineAnsiState = nil
@@ -476,6 +481,8 @@ func Run(opts *Options) (int, error) {
 							}, func() {
 								nthTransformer = newTransformer
 							})
+							terminal.ResumeRendering()
+							matcher.ResumeScan()
 							withNthChanged = true
 							bump = true
 						}

src/matcher.go

@@ -45,6 +45,8 @@ type Matcher struct {
 	slab           []*util.Slab
 	mergerCache    map[string]MatchResult
 	revision       revision
+	scanMutex      sync.Mutex
+	cancelScan     *util.AtomicBool
 }
 
 const (
@@ -66,7 +68,8 @@ func NewMatcher(cache *ChunkCache, patternBuilder func([]rune) *Pattern,
 		partitions:     partitions,
 		slab:           make([]*util.Slab, partitions),
 		mergerCache:    make(map[string]MatchResult),
-		revision:       revision}
+		revision:       revision,
+		cancelScan:     util.NewAtomicBool(false)}
 }
 
 // Loop puts Matcher in action
@@ -126,7 +129,9 @@ func (m *Matcher) Loop() {
 		}
 
 		if result.merger == nil {
+			m.scanMutex.Lock()
 			result = m.scan(request)
+			m.scanMutex.Unlock()
 		}
 
 		if !result.cancelled {
@@ -238,7 +243,7 @@ func (m *Matcher) scan(request MatchRequest) MatchResult {
 			break
 		}
 
-		if m.reqBox.Peek(reqReset) {
+		if m.cancelScan.Get() || m.reqBox.Peek(reqReset) {
 			return MatchResult{nil, nil, wait()}
 		}
 
@@ -269,6 +274,20 @@ func (m *Matcher) Reset(chunks []*Chunk, patternRunes []rune, cancel bool, final
 	m.reqBox.Set(event, MatchRequest{chunks, pattern, final, sort, revision})
 }
 
+// CancelScan cancels any in-flight scan, waits for it to finish,
+// and prevents new scans from starting until ResumeScan is called.
+// This is used to safely mutate shared items (e.g., during with-nth changes).
+func (m *Matcher) CancelScan() {
+	m.cancelScan.Set(true)
+	m.scanMutex.Lock()
+	m.cancelScan.Set(false)
+}
+
+// ResumeScan allows scans to proceed again after CancelScan.
+func (m *Matcher) ResumeScan() {
+	m.scanMutex.Unlock()
+}
+
 func (m *Matcher) Stop() {
 	m.reqBox.Set(reqQuit, nil)
 }

src/terminal.go

@@ -1738,6 +1738,17 @@ func (t *Terminal) Input() (bool, []rune) {
 	return paused, copySlice(src)
 }
 
+// PauseRendering blocks the terminal from reading items.
+// Must be paired with ResumeRendering.
+func (t *Terminal) PauseRendering() {
+	t.mutex.Lock()
+}
+
+// ResumeRendering releases the lock acquired by PauseRendering.
+func (t *Terminal) ResumeRendering() {
+	t.mutex.Unlock()
+}
+
 // UpdateCount updates the count information
 func (t *Terminal) UpdateCount(cnt int, final bool, failedCommand *string) {
 	t.mutex.Lock()