Visual artifact when closing X session

[MaxDamage]

Hi all. I've had this issue for some time now. When I close my X session, screen goes to black, but then, my wallpaper appears again for half a second, but wit a strange artifact into the top left corner. It looks like a rectangle made of more noisy rectangles. Then the screen goes black again and GDM starts, as usual.

When I tried an Ubuntu install I noticed it only appeared once I had installed the nvidia binary drivers. I think it could be an issue between framebuffer and the nvidia drivers. I'm using the latest nvidia ones, but it's happened with other versions. For framebuffer console, I use the vesa-tng one from standard gentoo-dev-sources. My X server is the latest stable X.org package in portage.

Anybody has this issue too? Any solution? Should I use another framebuffer driver? It looks very unpleasant. Thanx in advice.
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