Take the write mutex when clearing view lines and reading the buffer

A view's line buffer, its viewLines/tainted flags, and its hover state
are all written from the command-task goroutine (under writeMutex) as it
renders. But three accessors reached that same state from the UI thread
without the lock: SetView and the GUI-resize path cleared a view's lines
directly, viewsToRedrawContentOnly read the tainted flag, and Buffer read
the line buffer. Each raced a rendering task.

Guard them with writeMutex, matching the view's other buffer accessors.
These are reads/clears of state writeMutex already protects, not new
callers of it -- the view's geometry stays outside the mutex.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/gocui/gui.go

@@ -385,7 +385,7 @@ func (g *Gui) SetView(name string, x0, y0, x1, y1 int, overlaps byte) (*View, er
 		v.y1 = y1
 
 		if sizeChanged {
-			v.clearViewLines()
+			v.ClearViewLines()
 
 			if v.Editable {
 				cursorX, cursorY := v.TextArea.GetCursorXY()
@@ -1461,7 +1461,7 @@ func (g *Gui) flush() error {
 	// if GUI's size has changed, we need to redraw all views
 	if maxX != g.maxX || maxY != g.maxY {
 		for _, v := range g.views {
-			v.clearViewLines()
+			v.ClearViewLines()
 		}
 	}
 	g.maxX, g.maxY = maxX, maxY
@@ -1500,7 +1500,7 @@ func viewsToRedrawContentOnly(views []*View) []*View {
 	redrawIndexes := set.New[int]()
 
 	for i, v := range views {
-		if !v.tainted && !redrawIndexes.Includes(i) {
+		if !v.IsTainted() && !redrawIndexes.Includes(i) {
 			continue
 		}
 

pkg/gocui/view.go

@@ -214,6 +214,16 @@ func (v *View) clearViewLines() {
 	v.clearHover()
 }
 
+// ClearViewLines is clearViewLines guarded by writeMutex. It's for callers on
+// the UI thread (the layout pass) that touch a view whose content a task
+// goroutine may be writing concurrently: viewLines/tainted/hover are all
+// buffer state that writeMutex protects.
+func (v *View) ClearViewLines() {
+	v.writeMutex.Lock()
+	defer v.writeMutex.Unlock()
+	v.clearViewLines()
+}
+
 type searcher struct {
 	searchString       string
 	searchPositions    []SearchPosition
@@ -1287,6 +1297,8 @@ func (v *View) updateSearchPositions() {
 
 // IsTainted tells us if the view is tainted
 func (v *View) IsTainted() bool {
+	v.writeMutex.Lock()
+	defer v.writeMutex.Unlock()
 	return v.tainted
 }
 
@@ -1535,6 +1547,9 @@ func (v *View) BufferLines() []string {
 // Buffer returns a string with the contents of the view's internal
 // buffer.
 func (v *View) Buffer() string {
+	v.writeMutex.Lock()
+	defer v.writeMutex.Unlock()
+
 	return linesToString(v.lines)
 }