Followup to my message #10: The problem has disappeared for me after the recent (~1 week ago) kernel upgrade in testing. So it looks like this wasn't X's fault, but the kernel's fault. Or maybe both were involved.
So this is resolved for me. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
