Playing around with getting a scroll view to snap a side panel open or closed. `decelerationRate` is really cool!

SnappingSidePanel.swift

import UIKit

class ViewController: UIViewController, UIScrollViewDelegate {

    private let blueView = UIView()
    private let redView = UIView()
    private let scrollView = UIScrollView()

    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(scrollView)
        scrollView.addSubview(redView)
        scrollView.addSubview(blueView)

        scrollView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.decelerationRate = .fast
        scrollView.delegate = self

        redView.translatesAutoresizingMaskIntoConstraints = false
        redView.backgroundColor = #colorLiteral(red: 0.5725490451, green: 0, blue: 0.2313725501, alpha: 1)

        blueView.translatesAutoresizingMaskIntoConstraints = false
        blueView.backgroundColor = #colorLiteral(red: 0.2196078449, green: 0.007843137719, blue: 0.8549019694, alpha: 1)

        NSLayoutConstraint.activate([
            scrollView.leftAnchor.constraint(equalTo: view.leftAnchor),
            scrollView.rightAnchor.constraint(equalTo: view.rightAnchor),
            scrollView.topAnchor.constraint(equalTo: view.topAnchor),
            scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor),

            redView.heightAnchor.constraint(equalTo: view.heightAnchor),
            redView.widthAnchor.constraint(equalToConstant: 240),

            redView.leftAnchor.constraint(equalTo: scrollView.leftAnchor),
            redView.topAnchor.constraint(equalTo: scrollView.topAnchor),
            redView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor),
            redView.rightAnchor.constraint(equalTo: blueView.leftAnchor),

            blueView.heightAnchor.constraint(equalTo: view.heightAnchor),
            blueView.widthAnchor.constraint(equalTo: view.safeAreaLayoutGuide.widthAnchor),
            blueView.rightAnchor.constraint(equalTo: scrollView.rightAnchor),
            blueView.topAnchor.constraint(equalTo: scrollView.topAnchor),
            blueView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor),
        ])
    }

    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        let expandedX = scrollView.contentSize.width - scrollView.frame.width
        if velocity.x == 0 && (targetContentOffset.pointee.x == 0 || targetContentOffset.pointee.x == expandedX) {
            return
        } else if velocity.x < 0 {
            targetContentOffset.pointee = CGPoint.zero
        } else {
            targetContentOffset.pointee = CGPoint(x: expandedX, y: 0)
        }
    }
}