Swift开发之iOS11下UIToolBar非正常显示问题

在iOS11环境下，使用UITextField将UIDatePicker作为其inputView并设置一定高度，将UIToolbar作为其inputAccessoryView时，会出现UIToolbar非正常显示的问题；

实验环境如下：

IDE：XCode
系统：iOS11+
语言：Swift(OC同样会出现)
问题控件：UIToolbar

实验demo如下(Swift)：

import UIKit

class ViewController: UIViewController {

    lazy var textField: UITextField = {
        let textField_ = UITextField(frame: CGRect(origin: CGPoint.zero, size: CGSize(width: 200, height: 44)))
        textField_.center = self.view.center
        textField_.backgroundColor = UIColor.gray
        textField_.placeholder = "点击弹出键盘"
        return textField_
    }()

    lazy var textFieldInputView: UIView = {
        // 高度过低时，会出现其与UIToolbar之间间隔较大；高度过高时，UIToolbar无法显示
        let textFieldInputView_ = UIDatePicker(frame: CGRect(origin: CGPoint.zero, size: CGSize(width: 0, height: 250)))
        textFieldInputView_.backgroundColor = UIColor.yellow
//        textFieldInputView_.autoresizingMask = .flexibleHeight
        return textFieldInputView_
    }()

    lazy var toolBar: UIToolbar = {
        let toolBar_ = UIToolbar(frame: CGRect(origin: CGPoint.zero, size: CGSize(width: 0, height: 35)))
        toolBar_.backgroundColor = UIColor.black
        toolBar_.tintColor = UIColor.blue

        let spaceItem = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.flexibleSpace, target: nil, action: nil)

        let doneItem = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.done, target: self, action: #selector(clickDoneButtonItem(sender:)))

        toolBar_.items = [spaceItem, doneItem]
        return toolBar_
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        view.addSubview(textField)
        textField.inputView = textFieldInputView
        textField.inputAccessoryView = toolBar
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

// MARK: - event response
extension ViewController {
    @objc private func clickDoneButtonItem(sender: UIBarButtonItem) {
        textField.resignFirstResponder()
    }
}

当手动设置UIDatePicker高度时，会出现以下两种情况：