Why my Excel Drop-down menu (of data validation) is not scrolling

Question

having a bad moment with my Excel because the drop-down list of Data Validation in one specific spreadsheet of my workbook (DV works fine in other sheets though) is not scrolling at all.

For the said issue, I have done the following elimination methods to attempt to find out the root cause:

1. disabled all sheet-specific event procedures in VBA.
2. checked/unchecked the 'Zoomo on roll with IntelliMouse' in the advanced setting as per some online recommended remedy
3. re-directed the source in Data Validation to a shorter list of references to check if the current list (of 140 choices) is causing the problem of the static drop-down menu bar
4. checked if there are overlapping objects that somehow may be causing the issue.

Having done all the above mentioned, I cannot seem to resolve the issue in hand, although i can use the up/down arrow keys on my keyboard to navigate the choices within the drop-down menu.

so frustrating...