Not sure which daily build this started with, but for some time now Mouse scrolling closes drop-down menus. Also with a closed (non-expanded) drop-down the it doesn't scroll item by item.
I've gotten this for awhile too, MS wireless mouse with Intellipoint 8 drivers. Using a generic Dell branded mouse the dropdowns scroll fine however. Win 7 64-bit
In the interest of this stuff showing up in search for the next guy who runs across it:
Tried a Logitech MX705 with the default microsoft drivers (HID-compliant mouse) and boxes scroll fine. I suppose I could test this further by uninstalling Intellipoint and seeing if the default drivers enable scrolling with the MS mouse...