In Windows Look and Feel, ALT + Up/Down arrow is not expanding/collapsing the JCombobox's list

Steps to reproduce:

Run the attached test case,

1. press Alt/Down arrow on the Combobox to expand, and show the contents of the list.
2. press Alt/Up arrow on the Combobox to hide the contents.

expected: should expand/hide.
Actual: Does not expand/hide.

Fails with 5.0u12 b01. Working ok and passes in 6u1-b04.

Workaround: Mouse could be used to expand/hide the list instead of the keyboard shortcuts.