Problem with mouseenter

Problem with mouseenter

I have a div assign as 
height:40 
width:100%
position absolute 
top:0px 

When I go to do a mouseenter it is only activating when I get to the very top of the div. On the border ...
What am I doing wrong ...
Im using this to stop some css hover activity overlaying my text fields

$("#cssmenu").on("mouseenter", function(e){   
   $("#cssmenu").css("zIndex", 9999);         
   console.log('cssmenu enter');              
   $("#S1").css("zIndex", -1);                
   $("#tabs").css("zIndex", -1);              
 });