Please notive the hover effect for the ComboBox items.

With Qt 6.8 this works both for Window or ApplicationWindow as root.
With Qt 6.9 this does not hover with ApplicationWindow as root.

Tried on Windows + macOS (native styles).

import QtQuick
import QtQuick.Controls

// Window {
ApplicationWindow {
  width: 640
  height: 480
  visible: true
  title: qsTr("Hello World")

  ComboBox {
    model: 10
    width: 300
  }
}