Swift implementation to highlight Cocoa UI elements (http://stackoverflow.com/a/25984748/316803)

highlight.swift

    // Taken from:
    // https://gist.github.com/joelcox/28de2f0cb21ea47bd789

    NSColor.selectedMenuItemColor().set()
    NSRectFillUsingOperation(dirtyRect, .CompositeSourceOver);

    if (dirtyRect.size.height > 1) {
      let heightMinus1 = dirtyRect.size.height - 1
      let currentControlTint = NSColor.currentControlTint()
      let startingOpacity: CGFloat = currentControlTint == .BlueControlTint ? 0.16 : 0.09

      let gradient = NSGradient(startingColor: NSColor(white: CGFloat(1.0), alpha:startingOpacity), endingColor:NSColor(white: CGFloat(1.0), alpha: 0.0))!
      let startPoint = NSMakePoint(dirtyRect.origin.x, dirtyRect.origin.y + heightMinus1)
      let endPoint = NSMakePoint(dirtyRect.origin.x, dirtyRect.origin.y + 1)
      gradient.drawFromPoint(startPoint, toPoint: endPoint, options:NSGradientDrawsBeforeStartingLocation)

      if currentControlTint == .BlueControlTint {
        NSColor(white: CGFloat(1.0), alpha: CGFloat(0.1)).set()

        let smallerRect = NSMakeRect(dirtyRect.origin.x, dirtyRect.origin.y + heightMinus1, dirtyRect.size.width, CGFloat(1.0))
        NSRectFillUsingOperation(smallerRect, .CompositeSourceOver)
      }
    }

Updated for Swift 5 and modern types: https://gist.github.com/soffes/6a305de23f5663cf0e27f4b338606a3b

This doesn't match on Mojave though.