This corresponds to Pull Request #666 where the ability to select a Title/Prompt Text for the CheckComboBox was added.
With the addition of this feature, the title remains fixed even if there are item(s) selected, so there is no way of knowing if an item has been selected.
Some proposed solutions:
The way that it was before, with the item(s) showing and any text that was cut off being replaced with an ellipses. For example:
[Pizza, Hot Do...]
[Dog, Cat, Sna...]
Another proposed solution was to display the amount of item(s) that have been checked off. For example:
[3 of 3]
[4 of 7]
[5 of 6]
If no consensus can be reached, or for the sake of having different choices, perhaps the developer can be allowed to decide, via a method such as .setDisplayOption(Display.ITEMTEXT), which display to use?