From Cory Riddell, "The spherical manipulator uses a middle drag event for panning. It was

panning for left-right drags, but was zooming for up-down drags. I
changed it to pan both directions as I believe that is the correct
behaviour. If the original combination pan-zoom behaviour was
intentional, please let me know."

src/osgGA/SphericalManipulator.cpp

@@ -363,7 +363,7 @@ bool SphericalManipulator::calcMovement()
         osg::Matrix rotation_matrix;
         rotation_matrix=osg::Matrixd::rotate(_elevation,-1,0,0)*osg::Matrixd::rotate(PI_2+_heading,0,0,1);
 
-        osg::Vec3 dv(dx*scale,dy*scale,0);
+        osg::Vec3 dv(dx*scale,0,dy*scale);
         _center += dv*rotation_matrix;
 
         return true;