AutoLayout with UILabel error

Hi, I'm trouble in making good motion between UILabel and UIButton when UIButton changes its height.

If you click UIButton to result in longer its height, UILabel(red) will be short.
Okay That's good.

But my question is that when UILabel(red) is going to be short why it's bottom anchor is moving shortly up and down ?
I just changed button height constraint when button is clicked. UILabel (red) bottom constraint is still self.view.bottom.anchor.

I questioned this problem to other developer, I get other solution using UIView instead of UILabel.
But they didn't know why UILabel was going to be somethings like error. They thought that was maybe because of UILabel's intrinsicContentSize .

But I still clearly don't know why was happened.
I would appreciate you if you give me clear answer. thanks!


here is my entire code.
you can run exactly just copy and paste.

Code Block swift 
import UIKit
class ViewController3 : UIViewController {
   
  var greenLabel = UILabel()
  var redLabel = UILabel()
  var button = UIButton(type : .custom)
   
  var buttonConstraints : NSLayoutConstraint?
  var isClicked = false
   
  override func viewDidLoad() {
    super.viewDidLoad()
     
     
    self.view.backgroundColor = .white
    self.view.addSubview(greenLabel)
    self.view.addSubview(redLabel )
    self.view.addSubview(button)
   
    button.backgroundColor = .blue
    redLabel.backgroundColor = .red
    greenLabel.backgroundColor = .green
    
    button.setTitle("I'm Button", for: .normal)
    button.addTarget(self, action: #selector(click), for: .touchUpInside)
     
    redLabel.translatesAutoresizingMaskIntoConstraints = false
    greenLabel.translatesAutoresizingMaskIntoConstraints = false
    button.translatesAutoresizingMaskIntoConstraints = false
     
     
    buttonConstraints = button.heightAnchor.constraint(equalToConstant: 50)
    
 
    NSLayoutConstraint.activate([
      buttonConstraints!,
      greenLabel.topAnchor.constraint(equalTo: self.view.topAnchor, constant: 100),
      greenLabel.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 10),
      greenLabel.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -10),
      greenLabel.heightAnchor.constraint(equalToConstant: 50),
       
      button.topAnchor.constraint(equalTo: self.greenLabel.bottomAnchor, constant: 50),
      button.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 10),
      button.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -10),
       
      redLabel.topAnchor.constraint(equalTo: self.button.bottomAnchor, constant: 50),
      redLabel.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 10),
      redLabel.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -10),
      redLabel.bottomAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.bottomAnchor),
    ])
  }
  @objc func click() {
     
    switch isClicked {
    case true :
       
      UIView.animate(withDuration: 0.5, delay: 0.0, animations: {
        self.buttonConstraints?.constant = 50
        self.view.layoutIfNeeded()
         
      }, completion: { complete in
        self.isClicked = false
         
      })
       
    case false :
       
      UIView.animate(withDuration: 0.5, delay: 0.0, animations: {
        self.buttonConstraints?.constant = 200
        self.view.layoutIfNeeded()
      }, completion : { complete in
        self.isClicked = true
      })
       
    }
  }
}