I managed to get the slider to show up, but it's not centered, its tiny, the label is so small that it's just showing up as "...", and it's inside the painting canvas... Right now I'm adding it to the UI by doing
root.getChildren().add(slider);
What type is root?
If you changed it to type BorderPane, then you should be able to use:
BorderPane
root.setTop(myHBox)
I have the same problem. The slider and the label are in the top but are not centered.