[nrkn]

Firefox 14.0.1, Windows 7 x64. Grey line is visible for a split second as page loads then disappears. EDIT: well now this is bizarre - it disappears and reappears as I scroll.

[steve-howard]

I'm on Firefox 15, Windows 8 preview x64. Gray line stays visible. Odd.

[nrkn]

http://imgur.com/dFXFa

There is a grey line in frames 1 and 3 but not in 2.

[Hopka]

Same here (also 14.0.1, Win7 x64), the grey line only appears when the triangle scrolls near the top or the bottom of the viewport. In the middle of the viewport it is gone.