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Square root symbol broken on demos (Chrome 32 on OS X 10.9) #744
Not only is the square root symbol not aligned with the horizontal line over the content, but the horizontal line is less thick — something you notice if the two happen to be almost aligned.
This might relate to #295.
The placement of the horizontal line has always been a source of difficulty, and the results you get change depending on the font size, the font used (STIX versus TeX), the browser, and the OS. Chrome has always been particularly troublesome in this respect, and I can produce examples that have the line too high, just right, and too low all in the same formula. So there is no easy fix for this. The measurements MathJax has access to don't have the required precision to get this exact in all cases.
MathJax does include a value that can be used to adjust the line, and I think it can be changed to improve the situation, but not fix it entirely.
The horizontal line is made from minus signs in the font being used (so that its thickness will better match the rest of the display), and the quality of the result depends on the quality of the font rendering, antialiasing, and even the position on the page (how close the positioning is to the pixel grid of the screen makes a big difference in the result). Again, Chrome has some long-standing font issues that exacerbate the problem.