I've just been notified that this has been marked as a duplicate of
Bug #41092, which is labeled "Closed" and whose comments suggest that the problem is either not reproducible or is fixed in version 6.0.1.
But that's not true! The problem is happening now using version 6.0.1 (OSS), and I can reproduce it at will with the Ajax client on Firefox 3.5.3, Chrome 3.0.195.21, IE 7.0.5730.13, and IE 8.0.6001.18813.
The bug is still alive and kicking!
... Ed