Rendering bug in Edge and IE

Created by: fjwright

When I view https://www.mathjax.org/#samples in Microsoft Edge on Windows 10 the "Double angle formula for Cosines" example has a dashed square box containing the text f() overlaying each opening parenthesis and the following couple of characters; see below. If I use IE then the maths is not rendered at all. It all looks fine in Firefox and Chrome.

I initially noticed this problem on a web server that I manage. Everything is fine with an earlier version of MathJax, but when I experimented with the latest version I noticed this overlay bug in both Edge and IE (using the necessary polyfill).