Created attachment 117886 [details]
screenshot with TextDemo
Here's another interesting but minor issue. With Java 8 (unsure about earlier versions), some of the GUI text characters are displayed with yellow backgrounds--specifically the otherwise-transparent rectangular area containing each individual glyph. This may not apply to all Java GUI text classes, but seems to apply in the examples I've tried which use Swing components (so there's more I can do to narrow down which Swing/awt classes are affected, and possibly to identify which painting operations are responsible).
Attached is a screenshot with TextDemo (cf. https://docs.oracle.com/javase/tutorial/uiswing/components/textfield.html ) running on the right of the screen (sorry for not cropping--on the left is lxterm window with usual corruption I described in bug 88767).
I'm still using xf86-video-r128 6.9.2 at the moment, so this might need to be left as "NEEDINFO" until I get to try 6.10.0 to see if that makes a difference.
Does the problem go away with Option "ExaNoComposite" "true" in xorg.conf?
I reinstalled Debian on the system recently, and failed to reproduce this issue regardless of the ExaNoComposite option, but I'm using a different desktop environment this time (MATE instead of LXDE), and am currently using an older kernel version than I would have. So there's probably more specific conditions that this happens under.
Sad news: my iMac G3 is gone, but so was any free time to mess with it due to school. :^( Thanks for helping it run desktop Linux nearly 18 years after its manufacturing. My r128 issues can be RESOLVED as WONTFIX/WORKSFORME if appropriate.
Note the similarity between this bug, bug 88767, and bug 94913;
closing as RESOLVED/WONTFIX as suggested.