White Square Flashes Briefly in Corner - Chrome, Scrollbar-related?

This is odd:

A small white square will briefly flash into the upper-left corner of my browser window when I refresh a page using Chrome 11开发者_运维问答 on OS X.

I noticed the white box appears as the scrollbar renders.

As a test, I set overflow-y:hidden on the html element.

Sure enough, the white box no longer flashed as I refreshed the page.

Anybody know what would be causing this?

Here's a 32kb Quicktime:

http://ge.tt/33gTOk7