I think I found a bug in the combobox widget.
To make a long story short, please just have a look at your own example under:

1.) Just select any time.
2.) Click outside, so the timepicker has no focus.
3.) Click on the little arrow of the timepicker. You cannot select any other times any more.

(I found this problem with various comboboxes in my project too, but the timepicker in your example is just the perfect thing to show you the problem, so I am not posting any custom code here)

Thank you,
Bernhard Schulz