That may be a browser bug. Which browser and version were you using? Did you have the same problem in another browser or version of the same browser? The best way to move a menu in Sharepoint is the give the parent element "position: relative" and then give the menu "position: absolute" with relevant top and left values. This will move it relevant to its original position without causing damage to the layout, well, in general. It really depends on what your already have going on the page.