There is an issue with ComboBoxes where they will get stuck in an endless focus traversal loop that will force the user to have to kill the application. To see the issue, load up the attached fxml file in SceneBuilder and preview it. The focus will be on the left ComboBox. Click away from the application onto SceneBuilder to take the focus away from the previewed application. Click back on the right ComboBox (don't click on any other part of the application first) and the app will be frozen.