Thanks Lukasz for checking.
I downloaded the same twbx from this page and still see lines.
I asked a colleague to check and he does not see lines either so appears to be a problem with my installation even though we are both on the same version - 10.0.2 (10000.16.1004.1720) 64-bit
I'll get in touch with Tableau support or maybe uninstall and reinstall to see if that resolves the issue.
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After help and guidance from Tableau support I updated the Intel graphics drivers on my PC and this resolved the issue. I now have a nice clean polygon when using the same workbook!