border-top-width on body element causes incorrect display of lightbox in iOS Safari in Viewer

[ericfs]

Example page with issue:
https://www.google.com/amp/mashable-amphtml.googleusercontent.com/amphtml/pages/CAIiEAkT4PrPGjd0a1zZrN_xpVQqIggEKhoIACIQd3d3Lm1hc2hhYmxlLmNvbSoECAowCDCBmB4#

Tap the image to open the lightbox

What browsers are affected?

iOS Safari

Which AMP version is affected?

1463506087793

Is this a new issue? Or was it always broken? Paste your AMP version. You can find it in the browser dev tools.
I believe this is regression, but I'm not sure and I don't know when it first happened.

[erwinmombay]

/to @dvoytenko , feel free to reassign