A replacement for Cocoa’s NSSplitView. There are some serious limitations with NSSplitView if you need to limit subview’s sizes, expand or collapse subviews programmatically or by double-clicking, or resize the split view frequently. RBSplitView has special content views — RBSplitSubviews — that handle details of subview limitations and properties. So there’s less or no work to be done by the delegate. RBSplitView also has built-in support for nesting any number of levels, and automatically generates a two-axis thumb to resize in two dimensions.

Official RBSplitView web site