Why does the top left corner sometimes appear blank?

Occasionally, the top left corner of my Safari page will blank out on me (either keeping a previous window open - for instance while I'm composing mail in Gmail - I'll see the compose window for 3/4 of my screen but the inbox part on the top left part - OR nothing at all, just white - then I struggle to read what I've typed in the first few sentences, or the address bar, until I reach the center or right part of the screen). Does anyone else experience this? I've never had this problem on a previous mac, but I've had it routinely on this one (MBPw/R). Also, I don't use other browsers with any regularity, so I'm not sure if it happens elsewhere.

MBP w/Retina (2.3GHz, 16GB memory)

OS X 10.8.2

Safari 6.0.2