How do I control or fix the width of the pop up?

Did you were able to fix it? Having the same issue with any dropdown item inside a form. Edit: Even if they are outside of the form, they use the size of a whole other window.

@Luukas Unfortunately not, I encountered it when trying to bind the width of the ComboBox to the row width of a GridPane. When the width increases, the pop up and the ComboBox follow, when it decreases, only the ComboBox follows.