UIView.animate grows the rectangle from bottom-left to top-right instead of straight up vertically

Question

I am creating a pulldown menu using a stackview in conjunction with a textfield. When the textfield is tapped, I animate the contents of the stackview by "growing" the stackview upwards, but it is doing it from the bottom-left to the top-right corner. How can I make it "grow" straight up vertically upwards? I suspect that it may be due to the width of the buttons not being equal to the width of the stackview, but I have already set the stackview's alignment property to .fill as well as set a constraint to make the stackview's width anchor equal to the textfield's width anchor. So, I am not sure what I need to fix. Here's the code followed by a clip of how it looks right now.

import UIKit

class ViewController: UIViewController, UITextFieldDelegate {
    @IBOutlet weak var textfield: UITextField!
    @IBOutlet weak var label: UILabel!
    
    var popup: UIStackView = {
        let selectionList = ["Apple",
                             "Grapefriut",
                             "Peach",
                             "Orange",
                             "Pear ",
                            ]
        let pop = UIStackView()
        pop.axis = .vertical
        pop.spacing = 0
        pop.distribution = .fillEqually
        pop.alignment = .fill
        pop.layer.borderWidth = 1
        pop.layer.borderColor = UIColor.darkGray.cgColor
        pop.layer.cornerRadius = 6
        pop.clipsToBounds = true

        for selection in selectionList {
            let button = UIButton()
            button.setTitle(selection, for: .normal)
            button.backgroundColor = #colorLiteral(red: 0.8202667832, green: 0.9491913915, blue: 1, alpha: 1)
            button.setTitleColor(.darkGray, for: .normal)
            button.titleLabel?.font = UIFont.systemFont(ofSize: 14)
            button.addTarget(self, action: #selector(selectionTapped), for: .touchUpInside)
            button.isHidden = true
            pop.addArrangedSubview(button)
        }

        return pop
    }()
        
    override func viewDidLoad() {
        super.viewDidLoad()
        textfield.delegate = self
        
        let imageView = UIImageView(image: UIImage(named: "downarrow20x20"))
        imageView.contentMode = .center
        imageView.frame = CGRect(x: 0, y: 0, width: imageView.image!.size.width + 20.0, height: imageView.image!.size.height)
        textfield.rightView = imageView
        textfield.rightViewMode = .always
        textfield.inputView = UIView() // disable keyboard from popping up
        
        view.addSubview(popup)
        
        popup.translatesAutoresizingMaskIntoConstraints = false
        popup.bottomAnchor.constraint(equalTo: textfield.bottomAnchor).isActive = true
        popup.widthAnchor.constraint(equalTo: textfield.widthAnchor).isActive = true
        popup.centerXAnchor.constraint(equalTo: textfield.centerXAnchor).isActive = true
    }
    
    func textFieldDidBeginEditing(_ textField: UITextField) {
        showHidePopupContents(hide: false)
    }
    
    private func showHidePopupContents(hide: Bool) {
        UIView.animate(withDuration: 0.3) {
            self.popup.subviews.forEach { btn in
                btn.isHidden = hide
            }
        }
    }
    
    @objc func selectionTapped(_ button: UIButton) {
        if let buttonStr = button.title(for: .normal) {
            let str = buttonStr.trimmingCharacters(in: .whitespacesAndNewlines)
            label.text = "\(str) selected"
            textfield.text = str
        }

        // get focus out of textfield
        textfield.isEnabled = false
        textfield.isEnabled = true
        
        showHidePopupContents(hide: true)
    }
}