NSLog(@"%@", [self.testTextView.window.contentView superview].subviews);

2012-08-02 15:59:30.145 Example[16702:303] (
    "<_NSThemeCloseWidget: 0x100523dc0>", // the close button
    "<_NSThemeWidget: 0x100525ce0>", // the minimize button?
    "<_NSThemeWidget: 0x100524e90>", // the maximize button?
    "<NSView: 0x100512ad0>", // the content view
    "<__NSInspectorBarView: 0x100529d50>", // the inspector view
    "(<NSToolbarView: 0x10054e650>: FD2E0533-AB18-4E7E-905A-AC816CB80A26)" // the toolbar
)

NSArray *topLevelViews = [self.testTextView.window.contentView superview].subviews;
NSUInteger indexOfContentView = [topLevelViews indexOfObject:self.testTextView.window.contentView];
if (indexOfContentView + 1 < topLevelViews.count) {
    NSView *inspectorBar = [topLevelViews objectAtIndex:indexOfContentView + 1];
    NSLog(@"%@", inspectorBar);
}
NSLog(@"%@", topLevelViews);

NSView *inspectorBarView = nil;
for (NSView *topLevelView in topLevelViews) {
    if ([topLevelView isKindOfClass:NSClassFromString(@"__NSInspectorBarView")]) {
        inspectorBarView = topLevelView;
    }
}
NSLog(@"%@", inspectorBarView);

if (inspectorBarView) {
    NSRect newFrame = inspectorBarView.frame;
    newFrame.origin = NSZeroPoint;
    [inspectorBarView setAutoresizingMask:NSViewMaxYMargin | NSViewMaxXMargin];
    [inspectorBarView setFrame:newFrame];
}

if window.titlebarAccessoryViewControllers.count > 0 {
        let textViewInspectorBar = self.titlebarAccessoryViewControllers[0].view
        let inspectorBarHeight: CGFloat = textViewInspectorBar!.frame.height // 26.0 pt
    }

func tabView(_ tabView: NSTabView, didSelect tabViewItem: NSTabViewItem?) {
  if let inspectorBar = window!.titlebarAccessoryViewControllers.first(where:
    {$0.view.className == "__NSInspectorBarView"} )?.view {

    // move content view back to where it should be
    var sz = window!.contentView!.frame.size
    sz.height = window!.frame.size.height - 21
    window!.contentView?.setFrameSize(sz)

    // put the inspector where we want it
    inspectorBar.removeFromSuperview()
    let y = textPage.frame.size.height - inspectorBar.frame.size.height - 10
    inspectorBar.setFrameOrigin(NSPoint(x: 0, y: y))
    textPage.subviews.insert(inspectorBar, at: 0)
  }
}