diff --git a/libs/canvas/canvas/rect_set.h b/libs/canvas/canvas/rect_set.h
new file mode 100644
index 0000000000..fc03a52190
--- /dev/null
+++ b/libs/canvas/canvas/rect_set.h
@@ -0,0 +1,68 @@
+/*
+ * Copyright (C) 2012 Carl Hetherington <carl@carlh.net>
+ * Copyright (C) 2013-2014 Paul Davis <paul@linuxaudiosystems.com>
+ * Copyright (C) 2015-2017 Robin Gareus <robin@gareus.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef __CANVAS_RECTSET_H__
+#define __CANVAS_RECTSET_H__
+
+#include <vector>
+
+#include "canvas/item.h"
+#include "canvas/visibility.h"
+
+namespace ArdourCanvas {
+
+class LIBCANVAS_API RectSet : public Item
+{
+public:
+	RectSet (Canvas*);
+	RectSet (Item*);
+
+	void compute_bounding_box () const;
+	void render (Rect const & area, Cairo::RefPtr<Cairo::Context>) const;
+
+	bool covers (Duple const &) const;
+
+	void begin_add ();
+	void end_add ();
+
+	struct ResetRAII {
+		ResetRAII (RectSet& rs) : rects (rs) { rects.clear(); rects.begin_add(); }
+		~ResetRAII () { rects.end_add (); }
+		RectSet& rects;
+	};
+
+	void add_rect (int index, Rect const &, Gtkmm2ext::Color);
+	void clear ();
+
+	struct ColoredRectangle : Rect {
+		ColoredRectangle (int i, Rect const & ra, Gtkmm2ext::Color color_) : Rect (ra), index (i), color (color_) {}
+		int index;
+		Gtkmm2ext::Color color;
+	};
+
+	std::vector<ColoredRectangle> const & rects() const { return _rects; }
+
+private:
+	std::vector<ColoredRectangle> _rects;
+};
+
+}
+
+#endif /* __CANVAS_RECTSET_H__ */
diff --git a/libs/canvas/rect_set.cc b/libs/canvas/rect_set.cc
new file mode 100644
index 0000000000..3b9dae5ccc
--- /dev/null
+++ b/libs/canvas/rect_set.cc
@@ -0,0 +1,134 @@
+/*
+ * Copyright (C) 2012 Carl Hetherington <carl@carlh.net>
+ * Copyright (C) 2013-2017 Paul Davis <paul@linuxaudiosystems.com>
+ * Copyright (C) 2015-2017 Robin Gareus <robin@gareus.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <cmath>
+
+#include "canvas/rect_set.h"
+
+using namespace std;
+using namespace ArdourCanvas;
+
+RectSet::RectSet (Canvas* c)
+	: Item (c)
+{
+
+}
+
+RectSet::RectSet (Item* parent)
+	: Item (parent)
+{
+
+}
+
+void
+RectSet::compute_bounding_box () const
+{
+	if (_rects.empty ()) {
+		_bounding_box = Rect ();
+	} else {
+		Rect rb;
+
+		for (auto const & r : _rects) {
+			rb = rb.extend (r);
+		}
+
+		_bounding_box = rb;
+	}
+
+	set_bbox_clean ();
+}
+
+void
+RectSet::render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const
+{
+	/* area is in window coordinates */
+
+	Rect p (parent()->item_to_window (parent()->bounding_box()).intersection (area));
+
+	for (auto const & r : _rects) {
+
+		Rect self;
+
+		self = item_to_window (r, false);
+		Rect intersection = self.intersection (p);
+
+		if (!intersection) {
+			continue;
+		}
+
+		Gtkmm2ext::set_source_rgba (context, r.color);
+
+		/* XXXX another example of where using COORD_MAX fails
+		 * (presumably at some point Cairo folds from double to an
+		 * integral and overflows). Use a "big number" that isn't too
+		 * big.
+		 *
+		 * We should really find out what the upper limit actually is
+		 * and use that for COORD_MAX.
+		 */
+
+		context->rectangle (intersection.x0, intersection.y0, intersection.width(), intersection.height());
+		context->fill ();
+	}
+}
+
+void
+RectSet::add_rect (int index, Rect const & ra, Gtkmm2ext::Color color)
+{
+	_rects.push_back (ColoredRectangle (index, ra, color));
+}
+
+void
+RectSet::begin_add ()
+{
+	begin_change ();
+}
+
+void
+RectSet::end_add ()
+{
+	set_bbox_dirty ();
+	end_change ();
+}
+
+void
+RectSet::clear ()
+{
+	begin_change ();
+	_rects.clear ();
+	set_bbox_dirty ();
+	end_change ();
+}
+
+bool
+RectSet::covers (Duple const & point) const
+{
+	if (bounding_box().contains (point)) {
+		return true;
+	}
+
+	for (auto const & r : _rects) {
+		if (r.contains (point)) {
+			return true;
+		}
+	}
+
+	return false;
+}