Go to the MathML torture test with Linux and check test 13. If you select "Cambria Math" and then go back to the "Default Font", the size of the radical signs is not updated. That seems to be a problem with how the margin-top and stretch size are managed but I could not find exactly where it comes from when working on bug 119038.
BTW, I think there is actually a small "mistake" in RenderMathMLRoot:
should really be radicalWrapper()->... i.e. we should apply the margin to the wrapper, and leave the inner <mo> renderer alone. However, that does not change the problem here.
Created attachment 233292 [details]
(This also happens if you zoom in and out)
This bug will be fixed after the RenderMathMLRoot refactoring.
I verified that this is fixed by the http://trac.webkit.org/changeset/202168