I'm still waiting for SebK to confirm that he did zoom to get this error... The whole menu is made with background images and when chrome zoom is used all those background images are misplaced for 1px, that's why the white line appears... not sure how and if to fix this as this is obvious a chrome issue...
this is a solution: adding a 1px border to the menu will fullfill this empty space.
// original file is: http://www.yiiframew.../css/layout.css
// applies only to chrome
.layout-main-header .menu li
border-right: 1px solid #999;
border: 1px solid #999; // THIS LINE SOLVE THE PROBLEM WHEN CHROME ZOOMS TO 50%