Position UIPopover from NSToolbarItem

NSToolbarItem+UIPopoverPosition.swift

// You may want to present a UIKit popover from an NSToolbarItem, e.g. Maps.app uses
// a popover to select between viewing modes of default, Transit and Satlite.
//
// In UIKit land, set the popover controller's source view as the containing UIWindow,
// and use the following to grab the source rect from the NSToolbarItem in an AppKit
// bundle.
//
// Uses private APIs so mileage varies.
func position(for toolbarItem: NSToolbarItem) -> CGRect? {
    guard let view = toolbarItem.value(forKey: "_view") as? NSView,
          let window = view.value(forKey: "_window") as? NSWindow else {
        assertionFailure("Looks like your toolbar item isn't in the view hierarchy, or the private API changed.")
        return nil
    }

    var rect = view.convert(view.bounds, to: nil)
    
    // AppKit coordinate system is bottom upwards, whereas UIKit coordinate system is 
    // top downwards, so we need to flip the y-axis w.r.t. the item's height.
    let windowHeight = window.frame.height
    rect.origin.y = windowHeight - rect.maxY

    return rect
}