将NSAttributedString与多个frontendColor一起使用会使UILabel中的文本看起来比预期的低

Question

演示

https://github.com/NSFish/NSAttributedStringWithMultipleForegroundColor

说明

在这里，我有一个尽可能简单的UILabel

let label = UILabel()
label.backgroundColor = UIColor.gray
label.numberOfLines = 2
view.addSubview(label)
// autolayout stuff..

然后配置NSAttributedString以满足设计者的意图

let content = " 自营  回力/WARRIOR 轮胎 215/65R16 98H SR1"
let attributedText = NSMutableAttributedString.init(string: content)
let font = UIFont.systemFont(ofSize: 16)
let baselineOffset = (lineHeight - font.lineHeight) / 4
let attributes: [NSAttributedString.Key : Any] = [NSAttributedString.Key.font: font,
                                                  NSAttributedString.Key.foregroundColor: UIColor.black,
                                                  NSAttributedString.Key.baselineOffset: Int(baselineOffset)]

attributedText.setAttributes(attributes, range: NSRange.init(location: 0, length: content.count))

let paragraphStyle = NSMutableParagraphStyle()
paragraphStyle.maximumLineHeight = lineHeight
paragraphStyle.minimumLineHeight = lineHeight
        attributedText.addAttribute(NSAttributedString.Key.paragraphStyle, value: paragraphStyle, range: NSRange.init(location: 0, length: content.count))

label.attributedText = attributedText

内容字符串出现在UILabel的中心，与预期的一样。但是当我更改前两个字符的前景色时，事情开始出错

// text looks lower
var prefixAttributes = attributes
prefixAttributes[NSAttributedString.Key.foregroundColor] = UIColor.white
attributedText.setAttributes(prefixAttributes, range: NSRange.init(location: 0, length: 5))
attributedText.setAttributes(attributes, range: NSRange.init(location: 5, length: content.count - 5))
// text in the center, works
// attributedText.setAttributes(attributes, range: NSRange.init(location: 0, length: content.count))

这里唯一的区别是将范围（0，5）的前景色更改为白色。

结果

问题

我对TextKit不太熟悉，但是我确实知道UILabel中文本的位置应该由baselineOffset决定，或者至少由某些东西决定，但是文本的前景色...