Math.sign not supported in IE11

[lbud]

A usage of Math.sign was just introduced in 7e254fa#diff-b3089558a25dee1e74f59aa1829490e4R115, and unrelatedly I turned on a Windows computer for the first time in years to debug some Windows bugs and found out Math.sign is not supported in IE11, so this breaks. (Here is a very complicated table that says as much.) We should replace it with boring-er Javascript until IE catches up 😞