[Menu] move from offsetWidth to getBoundingClientRect for positioning [was `.MathJax_Menu` does not exceed a `left` property of >627]

Created by: DizzyZane

Test case: http://dizzyzane.web.fc2.com/markdown/tests/tests.html & http://dizzyzane.web.fc2.com/markdown/tests/tests.html#top (same page, different colouration)

The menu will not go into the right half of the column on a 1280px wide monitor.

Chrome version: Version 51.0.2704.0 canary (64-bit)

Firefox: 45.0.1