From 792fe016e7bf260c84d974abcb41ab0c7dbc5cfe Mon Sep 17 00:00:00 2001
From: Paul Davis <paul@linuxaudiosystems.com>
Date: Thu, 5 Jun 2014 18:18:28 -0400
Subject: [PATCH] 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/canvas/scroll_group.h |  2 ++
 libs/canvas/scroll_group.cc       | 30 ++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/libs/canvas/canvas/scroll_group.h b/libs/canvas/canvas/scroll_group.h
index d33e9b9080..7652ae2e9b 100644
--- a/libs/canvas/canvas/scroll_group.h
+++ b/libs/canvas/canvas/scroll_group.h
@@ -40,6 +40,8 @@ class LIBCANVAS_API ScrollGroup : public Group
 	bool covers_canvas (Duple const& d) const;
 	bool covers_window (Duple const& d) const;
 
+	void render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const;
+
   private:
 	ScrollSensitivity _scroll_sensitivity;
 	Duple             _scroll_offset;
diff --git a/libs/canvas/scroll_group.cc b/libs/canvas/scroll_group.cc
index 615be8007c..0ce612018a 100644
--- a/libs/canvas/scroll_group.cc
+++ b/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)
 {