Background 'jitter' in Chrome on Windows

Problem

Possible bug? I mention it here in case it helps someone else.

Client reports that the background image is jittery/shakes on all pages. I thought it was an issue with how the background image was implemented (background-size: cover; -- applied to a div.) but changing this did not solve the issue.
Turns out disabling the Christmas Snow module sorted it out. Perhaps a conflict somewhere. I haven't had time to investigate further.

Client is using Chrome 15.0.874.106m on Windows 7.
I found it hard to replicate as I only saw the problem on login pages, nowhere else, and didn't see the problem in Parallels running Windows .