I use Xinerama with two Monitors at a Mobility Radeon X1300, because that Card istn't supported by the OpenSource-Driver I used ait-drivers but on the second Monitor the Cursor was always a bit white rectangle, I solved that Problem by using ~x86. But since a few days the Problem was there again. So I tested serveral Versions and figured out that it works only with 8.36.5. Reproducible: Always
Frankly, nothing we could fix w/ ati-drivers, sorry.
Same problem here. A workaround is to mask the version which are affected by this bug: $ echo "=x11-drivers/ati-drivers-8.39.4" >> /etc/portage/package.mask
Try w/ 8.40.4 or whatever other version that works for you; if none works, you'll have to complain to ATI as noted above.