In response to https://blog.plan99.net/reacts-tictactoe-tutorial-in-kotlin-javafx-715c75a947d2 which is a TicTacToe game written in Kotlin using JavaFX as the GUI library

tttoe_scalafx.scala

import scalafx.application.JFXApp
import scalafx.application.JFXApp.PrimaryStage
import scalafx.beans.binding.Bindings
import scalafx.beans.property.ObjectProperty
import scalafx.collections.ObservableBuffer
import scalafx.scene.Scene
import scalafx.scene.control.{Button, Label}
import scalafx.scene.layout._

object Main extends JFXApp {

  sealed abstract class Player(val label: String)
  case object X extends Player("X")
  case object O extends Player("O")
  case object Nope extends Player("")

  val lines = List(
    (0, 1, 2),
    (3, 4, 5),
    (6, 7, 8),
    (0, 3, 6),
    (1, 4, 7),
    (2, 5, 8),
    (0, 4, 8),
    (2, 4, 6)
  )

  case class Board(squares: Array[Player] = Array.fill(9)(Nope)) {
    def coordinates(x: Int, y: Int): Int = x + (y * 3)

    def apply(x: Int, y: Int): Player = squares(coordinates(x, y))

    def copyUpdate(x: Int, y: Int, content: Player): Board = copy(squares.updated(coordinates(x, y), content))

    def computeWinner: Player = {
      for ((a, b, c) <- lines if squares(a) != Nope && squares(a) == squares(b) && squares(a) == squares(c)) return squares(a)
      Nope
    }
  }

  val boards = new ObservableBuffer[Board]
  val gameMoves = new VBox

  boards.onChange(gameMoves.children = boards.indices.map { index =>
    val i = index + 1
    new Button(f"Move $i") {
      onAction = (_) => resetGameTo(i)
    }
  })
  boards += new Board


  private val players: Iterator[Player] = Iterator.continually(List(X, O)).flatten
  val currentPlayer: ObjectProperty[Player] = ObjectProperty(players.next)

  def resetGameTo(i: Int): Unit = {
    currentPlayer.value = if (i % 2 == 1) X else O
    boards.setAll(boards.take(i))
  }

  def gameGrid: GridPane = {
    val grid = new GridPane()
    for (x <- 0 until 3; y <- 0 until 3) {
      val button = new Button {
        maxHeight = Double.MaxValue; maxWidth = Double.MaxValue
        hgrow = Priority.Always; vgrow = Priority.Always

        text <== Bindings.createStringBinding(() => boards.last(x,y).label, boards)
        disable <== Bindings.createBooleanBinding(() => {val b = boards.last; b.computeWinner != Nope || b(x,y) != Nope}, boards)
        onAction = (_) => playMove(x, y)
      }
      grid.add(button, x, y)
    }
    grid
  }

  def playMove(x: Int, y: Int): Unit = {
    boards += boards.last.copyUpdate(x, y, currentPlayer.value)
    currentPlayer.value = players.next()
  }

  stage = new PrimaryStage {
    title = "TTToe with ScalaFX"
    width = 500
    height = 400
    scene = new Scene {
      root = new BorderPane {
        center = gameGrid
        right = new VBox {
          children = Seq(
            new Label {
              text <== Bindings.createStringBinding(() => boards.last.computeWinner match {
                case winner@(X | O) => s"Player ${winner.label} wins!"
                case Nope => s"Current player: ${currentPlayer.value.label}"
              }, boards)
            },
            new Label("Rewind game to"),
            gameMoves
          )
        }
      }
    }
  }
}