ADDITIONAL SYSTEM INFORMATION :
Windows 10
JRE 21

A DESCRIPTION OF THE PROBLEM :
Creating a tooltip with constructor new can lead to null pointer exceptions:
- call `new Tooltip()`
- tooltip does some initial CSS layouting
- for this, some internal node tries to get it's stylable parent
- Tooltip overwrites this with returning the global static variable `Tooltip.BEHAVIOUR.hoveredNode`, if it's not null

If, at the same time, the user hovers or un-hovers a node, bookkeeping of the parent hierarchy in the background thread can get corrupted, leading to a NPE.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
- run the example
- place your mouse at the border of the bottom of the label, so that you rapidly un-hover and hover the label
- observe a NPE



EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
no NPE
ACTUAL -
an NPE

---------- BEGIN SOURCE ----------
/*
 * This source file was generated by the Gradle 'init' task
 */
package org.example.app

import javafx.application.Application
import javafx.scene.Scene
import javafx.scene.control.Label
import javafx.scene.control.Tooltip
import javafx.scene.layout.Border
import javafx.scene.layout.VBox
import javafx.scene.paint.Color
import javafx.stage.Stage
import javafx.util.Duration
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.javafx.JavaFx
import kotlinx.coroutines.launch
import kotlinx.coroutines.yield
import kotlin.coroutines.EmptyCoroutineContext

class App: Application() {
  companion object {
    @JvmStatic
    fun main(args: Array<String>) {
      launch(App::class.java, *args)
    }
  }

  override fun start(stage: Stage) {

    val root = VBox()
    val scene = Scene(root)
    stage.scene = scene
    val text = Label()
    text.text = "some text"
    text.border = Border.stroke(Color.BLACK)
    val tooltip1 = Tooltip()
    tooltip1.showDelay = Duration(0.0)
    tooltip1.hideDelay = Duration(0.0)
    tooltip1.text = "tooltip1"
    text.tooltip = tooltip1
    root.children += text
    stage.show()
    val scope = CoroutineScope(EmptyCoroutineContext)
    // continuously resize text to force re-layout
    scope.launch(Dispatchers.JavaFx) {
      val h1 = 400.0
      val h2 = 410.0
      while (true) {
        yield()
        text.minHeight = h1
        text.maxHeight = h1
        stage.height = h1
        yield()
        text.minHeight = h2
        text.maxHeight = h2
        stage.height = h2
      }
    }
    scope.launch {
      while (true) {
        val tooltip2 = Tooltip()
      }
    }
  }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
create all tooltip instances on the JavaFX thread to ensure no concurrent update of the global static variable

FREQUENCY : occasionally


Root Cause:
navigating up the parent hierarchy during CSS processing in a background thread encounters a null parent, causing the NPE.