For next release we add the NSSplitViewControlMBS to our Xojo plugins with the attached NSSplitViewMBS class.

The control implements all the events you need, so you can draw your own knob if needed, get notified if the user resizes the sub views.

With the NSSplitViewMBS class, you can control the behavior. For example, you can decide whether the split is vertical or horizontal. And you add the controls to the splitview, which doesn't need to be just two.

Please try the new control and the class and let us know how well it works for you.