class Test: ApplicationTest() {
    ...
    private lateinit var textfield: TextField
    private lateinit var button: Button

    @Before
    fun setUpFx() {
        ...
        interact {
            ...
            textfield = from(view.root).lookup("#beep").query()
            button = from(view.root).lookup("#derp").query()
        }
    }
    ...

    @Test fun testTextfield() {
        clickOn(textfield).write("Something")

        assertEquals("Something", textfield.text)
    }

    @Test fun testButton() {
        assertTrue(button.isDisabled)
        clickOn(textfield).write("Something")

        assertTrue(!button.isDisabled)
        clickOn(button)
    }

}