Issue: 
Menu DIV drop down is behind another DIV with a lower z-index. Unless I set the z-index of the latter to -1, the DIV stays on top. 
 
Solution: 
The issue was actually with the parent menu DIV of the drop down. It did not have the tag: 
position: relative; 
 
The position tag must be set, and set to absolute, fixed, or relative to work. 
  
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