Added Area.Repaint() and implemented it on GTK+. Untested as of yet.

redo/area.go

@@ -37,6 +37,11 @@ type Area interface {
 	// It panics if width or height is zero or negative.
 	SetSize(width int, height int)
 
+	// Repaint marks the given rectangle of the Area as needing to be redrawn.
+	// The given rectangle is clipped to the Area's size.
+	// If, after clipping, the rectangle is empty, Repaint does nothing.
+	Repaint(r image.Rectangle)
+
 	// RepaintAll marks the entirety of the Area as needing to be redrawn.
 	RepaintAll()
 }

redo/area_unix.go

@@ -70,6 +70,14 @@ func (a *area) SetSize(width, height int) {
 	C.gtk_widget_set_size_request(a._widget, C.gint(a.width), C.gint(a.height))
 }
 
+func (a *area) Repaint(r image.Rectangle) {
+	r = image.Rect(0, 0, a.width, a.height).Intersect(r)
+	if r.Empty() {
+		return
+	}
+	C.gtk_widget_queue_draw_area(a._widget, C.gint(r.Min.X), C.gint(r.Max.Y), C.gint(r.Dx()), C.gint(r.Dy()))
+}
+
 func (a *area) RepaintAll() {
 	C.gtk_widget_queue_draw(a._widget)
 }