canvas: manually remove changes from 6f91dc0799 and implement same goal in a different way

The AudioClipEditor features a scroll bar that is a part of the canvas. Because scroll
groups are at the top level of a canvas, the scroll bar is necessary within a scroll
group, which causes it to get confused about the difference between its own
position within the canvas and that of the scroll group. This commit introduces
a per-Item flag, _scroll_translation, which is true by default. If false, the
item will not have coordinates translated to reflect scroll group position.

libs/canvas/canvas.cc

@@ -299,7 +299,6 @@ Canvas::window_to_canvas (Duple const & d) const
 {
 	ScrollGroup* best_group = 0;
 	ScrollGroup* sg = 0;
-	bool grabbed_item_inside = false;
 
 	/* if the coordinates are negative, clamp to zero and find the item
 	 * that covers that "edge" position.
@@ -333,8 +332,6 @@ Canvas::window_to_canvas (Duple const & d) const
 
 				best_group = sg;
 
-				grabbed_item_inside = check_grabbed_item_inside (sg);
-
 				if (sg->sensitivity() == (ScrollGroup::ScrollsVertically | ScrollGroup::ScrollsHorizontally)) {
 					/* Can't do any better than this. */
 					break;
@@ -343,7 +340,7 @@ Canvas::window_to_canvas (Duple const & d) const
 		}
 	}
 
-	if (best_group && (!have_grab() || grabbed_item_inside)) {
+	if (best_group && (!have_grab() || grab_can_translate ())) {
 		return d.translate (best_group->scroll_offset());
 	}
 
@@ -1518,13 +1515,14 @@ GtkCanvas::resize_handler ()
 }
 
 bool
-GtkCanvas::check_grabbed_item_inside (Item* possible_parent) const
+GtkCanvas::grab_can_translate () const
 {
 	if (!_grabbed_item) {
-		return false;
+		/* weird, but correct! */
+		return true;
 	}
 
-	return _grabbed_item->is_descendant_of (*possible_parent);
+	return _grabbed_item->scroll_translation ();
 }
 
 /** Create a GtkCanvaSViewport.

libs/canvas/canvas/canvas.h

@@ -92,7 +92,7 @@ public:
 	virtual void unfocus (Item*) = 0;
 
 	virtual bool have_grab() const { return false; }
-	virtual bool check_grabbed_item_inside (Item*) const { return false; }
+	virtual bool grab_can_translate () const { return true; }
 
 	void render (Rect const &, Cairo::RefPtr<Cairo::Context> const &) const;
 
@@ -218,7 +218,7 @@ public:
 	void unfocus (Item*);
 
 	bool have_grab() const { return _grabbed_item; }
-	bool check_grabbed_item_inside (Item*) const;
+	bool grab_can_translate () const;
 
 	Rect visible_area () const;
 	Coord width() const;

libs/canvas/canvas/item.h

@@ -285,6 +285,9 @@ public:
         bool resize_queued() const { return _resize_queued; }
         void queue_resize();
 
+	bool scroll_translation() const { return _scroll_translation; }
+	void disable_scroll_translation ();
+
         /* only derived containers need to implement this, but this
            is where they compute the sizes and position and their
            children. A fixed-layout container (i.e. one where every child
@@ -369,7 +372,8 @@ private:
 
 	std::string _tooltip;
 	bool _ignore_events;
-
+	bool _scroll_translation;
+	
 	void find_scroll_parent ();
 	void propagate_show_hide ();
 };

libs/canvas/item.cc

@@ -49,6 +49,7 @@ Item::Item (Canvas* canvas)
 	, _requested_width (-1)
 	, _requested_height (-1)
 	, _ignore_events (false)
+	, _scroll_translation (true)
 {
 	DEBUG_TRACE (DEBUG::CanvasItems, string_compose ("new canvas item %1\n", this));
 }
@@ -68,6 +69,7 @@ Item::Item (Item* parent)
 	, _requested_width (-1)
 	, _requested_height (-1)
 	, _ignore_events (false)
+	, _scroll_translation (true)
 {
 	DEBUG_TRACE (DEBUG::CanvasItems, string_compose ("new canvas item %1\n", this));
 
@@ -1342,3 +1344,9 @@ Item::set_pack_options (PackOptions po)
 	/* must be called before adding/packing Item in a Container */
 	_pack_options = po;
 }
+
+void
+Item::disable_scroll_translation ()
+{
+	_scroll_translation = false;
+}