Using a CPSplitView

var splitView = [[CPSplitView alloc] initWithFrame:CGRectMake(0.0, 0.0, CGRectGetWidth([contentView bounds]), CGRectGetHeight([contentView bounds]))];
[splitView setIsPaneSplitter: YES];
[splitView setVertical:YES];
[splitView setDelegate: nil];
[splitView setAutoresizingMask: CPViewWidthSizable | CPViewHeightSizable];
[splitView setBackgroundColor: [CPColor darkGrayColor]];
[contentView addSubview: splitView];

// Sidebar
var sidebar = [[SidebarView alloc] init:contentView];
[contentView addSubview: sidebar];