I am showing a frame with a button and a choice. On clicking the button, I am toggling the focusability of Choice by calling setFocusable(true) / (false). Initially Choice is focusable. When the frame is shown, click on the button. Choice becomes non-focusable. Try the following scenarios:

1. Click the Choice. Drop-down opens up. Move the mouse inside the drop-down and start moving the mouse wheel. No MouseWheel events are triggered and scrolling does not happen.

2. Click the choice. Drop-down opens. Keep the mouse pointer inside the component itself and move the mouse wheel. The items are scrolling but not properly. The items scroll automatically and come back to the same position where it was before moving the mouse wheel. In other words, the newly scrolled position resets to the default one automatically.

3. Do not click on Choice. Just move the mouse wheel keeping the mouse pointer inside the component. Items do not scroll but MouseWheel events are triggered.

All the above behaviors are noticed on WinXP with the following PIT build:
java version "1.6.0-internal"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-internal-java2d_09_sep_2005_03_26-b00)
Java HotSpot(TM) Client VM (build 1.6.0-ea-b51, mixed mode, sharing)

The above scenarios could not be tested on Mustang-b51 or before because the drop-down was not at all opening when clicking the choice when the choice is non-focusable. I don't think non-focusability should have any kind of impact on MouseWheel functionality.

Run the attached test and try exercising the scenarios given above.