Coalesce visual changes to canvas/items and allow canvas to render

First visual change will be processed as normal and then blocked until the
canvas renders the change. If further visual changes need processing then
Editor::pre_render callback will schedule another expose/redraw/render.

This prevents an issue where idle_visual_changer is called many times in
response to events(keys/motion/etc) but the canvas does not get a chance to
render any but the last one which results in a big pause/jump.

This results in a more responsive canvas and in particular a smoother and more
predictable zooming experience.

gtk2_ardour/editor.cc

@@ -345,6 +345,7 @@ Editor::Editor ()
 	, _full_canvas_height (0)
 	, edit_controls_left_menu (0)
 	, edit_controls_right_menu (0)
+	, visual_change_queued(false)
 	, _last_update_time (0)
 	, _err_screen_engine (0)
 	, cut_buffer_start (0)
@@ -4578,9 +4579,23 @@ Editor::_idle_visual_changer (void* arg)
 	return static_cast<Editor*>(arg)->idle_visual_changer ();
 }
 
+void
+Editor::pre_render ()
+{
+	visual_change_queued = false;
+
+	if (pending_visual_change.pending != 0) {
+		ensure_visual_change_idle_handler();
+	}
+}
+
 int
 Editor::idle_visual_changer ()
 {
+	if (pending_visual_change.pending == 0) {
+		return 0;
+	}
+
 	/* set_horizontal_position() below (and maybe other calls) call
 	   gtk_main_iteration(), so it's possible that a signal will be handled
 	   half-way through this method.  If this signal wants an
@@ -4592,6 +4607,11 @@ Editor::idle_visual_changer ()
 	*/
 
 	pending_visual_change.idle_handler_id = -1;
+
+	if (visual_change_queued) {
+		return 0;
+	}
+
 	pending_visual_change.being_handled = true;
 
 	VisualChange vc = pending_visual_change;
@@ -4602,6 +4622,8 @@ Editor::idle_visual_changer ()
 
 	pending_visual_change.being_handled = false;
 
+	visual_change_queued = true;
+
 	return 0; /* this is always a one-shot call */
 }
 

gtk2_ardour/editor.h

@@ -1148,6 +1148,9 @@ private:
 	};
 
 	VisualChange pending_visual_change;
+	bool visual_change_queued;
+
+	void pre_render ();
 
 	static int _idle_visual_changer (void *arg);
 	int idle_visual_changer ();

gtk2_ardour/editor_canvas.cc

@@ -241,6 +241,8 @@ Editor::initialize_canvas ()
 	_track_canvas->signal_enter_notify_event().connect (sigc::mem_fun(*this, &Editor::entered_track_canvas), false);
 	_track_canvas->set_flags (CAN_FOCUS);
 
+	_track_canvas->PreRender.connect (sigc::mem_fun(*this, &Editor::pre_render));
+
 	/* set up drag-n-drop */
 
 	vector<TargetEntry> target_table;