this is a solution: adding a 1px border to the menu will fullfill this empty space.
[color=#880000]// original file is: http://www.yiiframework.com/css/layout.css[/color][color=#000000]
[/color][color=#880000]// applies only to chrome[/color][color=#000000]
[/color][color=#880000]// [/color][color=#000000]
[/color][color=#666600].[/color][color=#000000]layout[/color][color=#666600]-[/color][color=#000000]main[/color][color=#666600]-[/color][color=#000000]header [/color][color=#666600].[/color][color=#000000]menu li
[/color][color=#666600]{[/color][color=#000000]
position[/color][color=#666600]:[/color][color=#000000]relative[/color][color=#666600];[/color][color=#000000]
display[/color][color=#666600]:[/color][color=#000000] [/color][color=#000088]inline[/color][color=#666600];[/color][color=#000000]
margin[/color][color=#666600]:[/color][color=#000000] [/color][color=#006666]0[/color][color=#666600];[/color][color=#000000]
padding[/color][color=#666600]:[/color][color=#000000] [/color][color=#006666]0[/color][color=#666600];[/color][color=#000000]
[/color][color=#000088]float[/color][color=#666600]:[/color][color=#000000] left[/color][color=#666600];[/color][color=#000000]
border[/color][color=#666600]-[/color][color=#000000]right[/color][color=#666600]:[/color][color=#000000] [/color][color=#006666]1px[/color][color=#000000] solid [/color][color=#880000]#999;[/color][color=#000000]
text[/color][color=#666600]-[/color][color=#000000]transform[/color][color=#666600]:[/color][color=#000000]lowercase[/color][color=#666600];[/color][color=#000000]
line[/color][color=#666600]-[/color][color=#000000]height[/color][color=#666600]:[/color][color=#000000] [/color][color=#006666]19.5px[/color][color=#666600];[/color][color=#000000]
border[/color][color=#666600]:[/color][color=#000000] [/color][color=#006666]1px[/color][color=#000000] solid [/color][color=#880000]#999; // THIS LINE SOLVE THE PROBLEM WHEN CHROME ZOOMS TO 50%[/color][color=#000000]
[/color][color=#666600]}[/color]