Update 1.0

But still - as you can see in the video - the scroll indicator bar shows that wrong heights were calculated:

So what you want is precise content height.

For that purpose, you cannot use static estimatedRowHeight. You should implement more correct estimation like below.

    ...

    var sampleCell: WorldMessageCell?

    override func viewDidLoad() {
        super.viewDidLoad()

        tableView.register(UINib(nibName: "WorldMessageCell", bundle: nil), forCellReuseIdentifier: "WorldMessageCell")

        sampleCell = UINib(nibName: "WorldMessageCell", bundle: nil).instantiate(withOwner: WorldMessageCell.self, options: nil)[0] as? WorldMessageCell
    }

    ...

    func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
        if let cell = sampleCell {
            let text = self.textForRowAt(indexPath)
            // note: this is because of "constrain to margins", which value is actually set after estimation. Do not use them to remove below
            let margin = UIEdgeInsets(top: 8, left: 20, bottom: 8, right: 20)
            // without "constrain to margins"
            // let margin = cell.contentView.layoutMargins 
            let maxSize = CGSize(width: tableView.frame.size.width - margin.left - margin.right,
                                 height: CGFloat.greatestFiniteMagnitude)
            let attributes: [NSAttributedString.Key: Any]? = [NSAttributedString.Key.font: cell.messageLabel.font]
            let size: CGRect = (text as NSString).boundingRect(with: maxSize,
                                                                 options: [.usesLineFragmentOrigin], attributes: attributes, context: nil)
            return size.height + margin.top + margin.bottom
        }
        return 100
    }

This is too precise (actually real row height) and maybe slow, but you can do more approximate estimation for optimization.

You need to set tableFooterView to empty.

override func viewDidLoad() {
    super.viewDidLoad()
    tableView.tableFooterView = UIView()
    // your staff
}

The problem is with your estimatedHeightForRowAt method. As the name implies it gives the estimated height to the table so that it can have some idea about the scrollable content until the actual content will be displayed. The more accurate value will result in a more smooth scrolling and height estimation.

You should set this value to big enough so that it can represent the height of your cell with the maximum content. In your case 650 is working fine.

func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
    return 650
}

The result would be far better with this approach.

Also, there is no need to implement delegate method for height until you want a variation on index bases. You can simply set table view property.

tableView.estimatedRowHeight = 650.0
tableView.rowHeight = .automaticDimension

Optimization

One more thing I noticed in your demo project. You've used too many if-else in your cellForRowAtIndexPath which is making it little slower. Try to minimize that. I've done some refinement to this, and it improves the performance.

1. Define an array which holds your message text.

   var messages = ["Lorem ipsum,"many more",.....]

2. Replace your cellForRowAt indexPath with below:

   func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell { var cell : WorldMessageCell cell = tableView.dequeueReusableCell(withIdentifier: "WorldMessageCell", for: indexPath) as! WorldMessageCell if indexPath.row < 14 { cell.messageLabel.text = messages[indexPath.row] } else if indexPath.row >= 14 && indexPath.row != 27 { cell.messageLabel.text = messages[14] } else if indexPath.row == 27 { cell.messageLabel.text = messages.last } return cell }