Put playhead on top of everything.

Achieve this by adding a new hscroll group just for cursors.

That requires a slightly smarter window_to_canvas() to deal with overlapping
sensitive scroll groups.  New rule is that scroll groups can overlap, but the
most sensitive one found from the top down will be chosen to translate
coordinates.  This basically means don't overlap scroll groups with different
sensitivities.

In the presence of scroll groups, having a canvas-wide window_to_canvas()
and/or canvas_to_window() fundamentally makes no sense.  At some point in the
glorious future we should kill those and use only item-relative coordinate
translation.

gtk2_ardour/editor.h

@@ -504,6 +504,7 @@ class Editor : public PublicEditor, public PBD::ScopedConnectionList, public ARD
 	ArdourCanvas::Container* get_noscroll_group () const { return no_scroll_group; }
 	ArdourCanvas::ScrollGroup* get_hscroll_group () const { return h_scroll_group; }
 	ArdourCanvas::ScrollGroup* get_hvscroll_group () const { return hv_scroll_group; }
+	ArdourCanvas::ScrollGroup* get_cursor_scroll_group () const { return cursor_scroll_group; }
 
 	ArdourCanvas::GtkCanvasViewport* get_track_canvas () const;
 
@@ -820,6 +821,10 @@ class Editor : public PublicEditor, public PBD::ScopedConnectionList, public ARD
 	*/
 	ArdourCanvas::ScrollGroup* h_scroll_group;
 
+	/* Scroll group for cursors, scrolled horizontally, above everything else
+	*/
+	ArdourCanvas::ScrollGroup* cursor_scroll_group;
+
 	/* The group containing all trackviews. */
 	ArdourCanvas::Container* no_scroll_group;
 

gtk2_ardour/editor_canvas.cc

@@ -80,6 +80,7 @@ Editor::initialize_canvas ()
 
 	ArdourCanvas::ScrollGroup* hsg; 
 	ArdourCanvas::ScrollGroup* hg;
+	ArdourCanvas::ScrollGroup* cg;
 
 	h_scroll_group = hg = new ArdourCanvas::ScrollGroup (_track_canvas->root(), ArdourCanvas::ScrollGroup::ScrollsHorizontally);
 	CANVAS_DEBUG_NAME (h_scroll_group, "canvas h scroll");
@@ -91,6 +92,10 @@ Editor::initialize_canvas ()
 	CANVAS_DEBUG_NAME (hv_scroll_group, "canvas hv scroll");
 	_track_canvas->add_scroller (*hsg);
 
+	cursor_scroll_group = cg = new ArdourCanvas::ScrollGroup (_track_canvas->root(), ArdourCanvas::ScrollGroup::ScrollsHorizontally);
+	CANVAS_DEBUG_NAME (cursor_scroll_group, "canvas cursor scroll");
+	_track_canvas->add_scroller (*cg);
+
 	_verbose_cursor = new VerboseCursor (this);
 
 	/* on the bottom, an image */

gtk2_ardour/editor_cursors.cc

@@ -33,7 +33,7 @@ using namespace Gtk;
 
 EditorCursor::EditorCursor (Editor& ed, bool (Editor::*callbck)(GdkEvent*,ArdourCanvas::Item*))
 	: _editor (ed)
-	, _track_canvas_item (new ArdourCanvas::Arrow (_editor.get_hscroll_group()))
+	, _track_canvas_item (new ArdourCanvas::Arrow (_editor.get_cursor_scroll_group()))
 {
 	CANVAS_DEBUG_NAME (_track_canvas_item, "track canvas editor cursor");
 

gtk2_ardour/public_editor.h

@@ -370,6 +370,7 @@ class PublicEditor : public Gtk::Window, public PBD::StatefulDestructible, publi
 	virtual ArdourCanvas::Container* get_trackview_group () const = 0;
 	virtual ArdourCanvas::ScrollGroup* get_hscroll_group () const = 0;
 	virtual ArdourCanvas::ScrollGroup* get_hvscroll_group () const = 0;
+	virtual ArdourCanvas::ScrollGroup* get_cursor_scroll_group () const = 0;
 
         virtual ArdourCanvas::GtkCanvasViewport* get_track_canvas() const = 0;
 

libs/canvas/canvas.cc

@@ -236,7 +236,7 @@ Canvas::window_to_canvas (Duple const & d) const
 	 */
 
 	std::list<Item*> const& root_children (_root.items());
-	ScrollGroup* sg = 0;
+	ScrollGroup* best_group = 0;
 
 	/* if the coordinates are negative, clamp to zero and find the item
 	 * that covers that "edge" position.
@@ -252,13 +252,20 @@ Canvas::window_to_canvas (Duple const & d) const
 	}
 
 	for (std::list<Item*>::const_reverse_iterator i = root_children.rbegin(); i != root_children.rend(); ++i) {
-		if (((sg = dynamic_cast<ScrollGroup*>(*i)) != 0) && sg->covers_window (in_window)) {
-			break;
+		ScrollGroup* sg = dynamic_cast<ScrollGroup*>(*i);
+		/* If scroll groups overlap, choose the one with the highest sensitivity,
+		   that is, choose an HV scroll group over an H or V only group. */
+		if (sg && (!best_group || sg->sensitivity() > best_group->sensitivity())) {
+			best_group = sg;
+			if (sg->sensitivity() == (ScrollGroup::ScrollsVertically | ScrollGroup::ScrollsHorizontally)) {
+				/* Can't do any better than this. */
+				break;
+			}
 		}
 	}
 
-	if (sg) {
-		return d.translate (sg->scroll_offset());
+	if (best_group) {
+		return d.translate (best_group->scroll_offset());
 	}
 
 	return d;

libs/canvas/canvas/scroll_group.h

@@ -46,6 +46,8 @@ class LIBCANVAS_API ScrollGroup : public Container
 
 	void render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const;
 
+	ScrollSensitivity sensitivity() const { return _scroll_sensitivity; }
+
   private:
 	ScrollSensitivity _scroll_sensitivity;
 	Duple             _scroll_offset;