clip rendering of ScrollGroup to just the part of the canvas covered by the scroll group.

This stops tracks from appearing to scroll up under the rulers, among other things.

libs/canvas/scroll_group.cc

@@ -20,6 +20,7 @@
 
 #include "pbd/compose.h"
 
+#include "canvas/canvas.h"
 #include "canvas/debug.h"
 #include "canvas/scroll_group.h"
 
@@ -38,6 +39,35 @@ ScrollGroup::ScrollGroup (Group* parent, Duple position, ScrollSensitivity s)
 {
 }
 
+void
+ScrollGroup::render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const
+{
+	/* clip the draw to the area that this scroll group nominally occupies
+	 * WITHOUT scroll offsets in effect
+	 */
+
+	boost::optional<Rect> r = bounding_box();
+
+	if (!r) {
+		return;
+	}
+
+	Rect self (_position.x, _position.y, _position.x + r.get().width(), _position.y + r.get().height());
+
+	self.x1 = min (_position.x + _canvas->width(), self.x1);
+	self.y1 = min (_position.y + _canvas->height(), self.y1);
+
+	context->save ();
+	context->rectangle (self.x0, self.y0, self.width(), self.height());
+	context->clip ();
+
+	Group::render (area, context);
+
+	context->restore ();
+
+
+}
+
 void
 ScrollGroup::scroll_to (Duple const& d)
 {