Make focus reporting an opt-in feature, like mouse reporting

_demos/mouse.go

@@ -129,6 +129,7 @@ func main() {
 	s.SetStyle(defStyle)
 	s.EnableMouse()
 	s.EnablePaste()
+	s.EnableFocus()
 	s.Clear()
 
 	posfmt := "Mouse: %d, %d  "

screen.go

@@ -139,6 +139,12 @@ type Screen interface {
 	// DisablePaste disables bracketed paste mode.
 	DisablePaste()
 
+	// EnableFocus enables reporting of focus events, if your terminal supports it.
+	EnableFocus()
+
+	// DisableFocus disables reporting of focus events.
+	DisableFocus()
+
 	// HasMouse returns true if the terminal (apparently) supports a
 	// mouse.  Note that the return value of true doesn't guarantee that
 	// a mouse/pointing device is present; a false return definitely

simulation.go

@@ -325,6 +325,12 @@ func (s *simscreen) DisablePaste() {
 	s.paste = false
 }
 
+func (s *simscreen) EnableFocus() {
+}
+
+func (s *simscreen) DisableFocus() {
+}
+
 func (s *simscreen) Size() (int, int) {
 	s.Lock()
 	w, h := s.back.Size()

tscreen.go

@@ -163,6 +163,7 @@ type tScreen struct {
 	wg           sync.WaitGroup
 	mouseFlags   MouseFlags
 	pasteEnabled bool
+	focusEnabled bool
 
 	sync.Mutex
 }
@@ -1056,6 +1057,20 @@ func (t *tScreen) enablePasting(on bool) {
 	}
 }
 
+func (t *tScreen) EnableFocus() {
+	t.Lock()
+	t.focusEnabled = true
+	t.enableFocusReporting()
+	t.Unlock()
+}
+
+func (t *tScreen) DisableFocus() {
+	t.Lock()
+	t.focusEnabled = false
+	t.disableFocusReporting()
+	t.Unlock()
+}
+
 func (t *tScreen) enableFocusReporting() {
 	if t.enableFocus != "" {
 		t.TPuts(t.enableFocus)
@@ -1866,7 +1881,9 @@ func (t *tScreen) engage() error {
 	t.stopQ = stopQ
 	t.enableMouse(t.mouseFlags)
 	t.enablePasting(t.pasteEnabled)
+	if t.focusEnabled {
 		t.enableFocusReporting()
+	}
 
 	ti := t.ti
 	t.TPuts(ti.EnterCA)