Add Widget.Clip

box.go

@@ -82,6 +82,13 @@ func (b *Box) SetVisible(visible bool) {
 	b.visible = visible
 }
 
+// Clip returns whether the widget and its children are restricted to drawing
+// within the widget's rect area of the screen. For best performance, Clip
+// should return false unless clipping is actually needed.
+func (b *Box) Clip() bool {
+	return false
+}
+
 // Cursor returns the cursor shape shown when a mouse cursor hovers over the
 // widget, or -1 to let widgets beneath determine the cursor shape.
 func (b *Box) Cursor() ebiten.CursorShapeType {

game.go

@@ -429,11 +429,18 @@ func draw(w Widget, screen *ebiten.Image) error {
 	}
 
 	r := w.Rect()
-	subScreen := screen.SubImage(r).(*ebiten.Image)
+	subScreen := screen
+	if w.Clip() {
+		subScreen = screen.SubImage(r).(*ebiten.Image)
+	}
 
 	background := w.Background()
 	if background.A > 0 {
-		subScreen.Fill(background)
+		if subScreen == screen {
+			screen.SubImage(r).(*ebiten.Image).Fill(background)
+		} else {
+			subScreen.Fill(background)
+		}
 	}
 
 	err := w.Draw(subScreen)

list.go

@@ -161,6 +161,13 @@ func (l *List) SetVisible(visible bool) {
 	l.grid.SetVisible(visible)
 }
 
+// Clip returns whether the widget and its children are restricted to drawing
+// within the widget's rect area of the screen. For best performance, Clip
+// should return false unless clipping is actually needed.
+func (l *List) Clip() bool {
+	return true
+}
+
 // SetColumnSizes sets the size of each column. A size of -1 represents an equal
 // proportion of the available space.
 func (l *List) SetColumnSizes(size ...int) {

select.go

@@ -58,6 +58,13 @@ func (s *Select) SetRect(r image.Rectangle) {
 	s.list.SetRect(listRect)
 }
 
+// Clip returns whether the widget and its children are restricted to drawing
+// within the widget's rect area of the screen. For best performance, Clip
+// should return false unless clipping is actually needed.
+func (s *Select) Clip() bool {
+	return false
+}
+
 // SetHighlightColor sets the color used to highlight the currently selected item.
 func (s *Select) SetHighlightColor(c color.RGBA) {
 	s.list.SetHighlightColor(c)

widget.go

@@ -34,6 +34,11 @@ type Widget interface {
 	// SetVisible sets the visibility of the widget.
 	SetVisible(visible bool)
 
+	// Clip returns whether the widget and its children are restricted to drawing
+	// within the widget's rect area of the screen. For best performance, Clip
+	// should return false unless clipping is actually needed.
+	Clip() bool
+
 	// Cursor returns the cursor shape shown when a mouse cursor hovers over
 	// the widget, or -1 to let widgets beneath determine the cursor shape.
 	Cursor() ebiten.CursorShapeType

window.go

@@ -55,6 +55,13 @@ func (w *Window) SetRect(r image.Rectangle) {
 	w.modified = true
 }
 
+// Clip returns whether the widget and its children are restricted to drawing
+// within the widget's rect area of the screen. For best performance, Clip
+// should return false unless clipping is actually needed.
+func (w *Window) Clip() bool {
+	return false
+}
+
 // SetFont sets the font and text size of the window titles. Scaling is not applied.
 func (w *Window) SetFont(fnt *text.GoTextFaceSource, size int) {
 	w.Lock()