Index: gtk2_ardour/stereo_panner.cc
===================================================================
--- gtk2_ardour/stereo_panner.cc	(revision 11752)
+++ gtk2_ardour/stereo_panner.cc	(working copy)
@@ -469,8 +469,8 @@
 		return false;
 	}
 
-	int w = get_width();
-	double delta = (ev->x - last_drag_x) / (double) w;
+	int usable_width = get_width() - lr_box_size;
+	double delta = (ev->x - last_drag_x) / (double) usable_width;
 	double current_width = width_control->get_value ();
 
 	if (dragging_left) {
@@ -503,7 +503,8 @@
 			}
 
 		} else {
-			width_control->set_value (current_width + delta);
+			/* width needs to change by 2 * delta because both L & R move */
+			width_control->set_value (current_width + delta * 2);
 		}
 
 	} else if (dragging_position) {