I'm running 10.8.2 and this just happened last week (end of January, 2013). I took a picture of my screen at it looked very much the same as your screenshot.
I was able to "recover" from the situation by keeping the external monitor attached, closing the "lid"/display of my
MacBookPro8,2, waiting for all windows to settle onto the external monitor, and then re-opening the laptop.
I'm not at all happy or satisfied with that result, but it seems to take care of it.
No firmware changes on my part. I was having other issues with scrolling in Mail.app that cropped up today.
I tried a suggestion that I found here: http://forums.macrumors.com/showthread.php?t=1430454
I went to System Preferences -> Displays and switched the laptop to a different resolution (which brought with it an adventure in seeing through screen gobbledy-gook) and then back to the full resolution capability of the LCD panel.
The mail scrolling issues went away and maybe this step lets someone track down what is going on.
As far as I know...
- workaround for gobbledy-gook on laptop display: close and re-open lid
- scrolling issue with Mail.app recovers after switching display resolution
Maybe they're related, maybe not.