I uninstalled vivaldi-preview_22.214.171.124-1_amd64 and then installed vivaldi-snapshot_126.96.36.199-1_amd64; thank you mib.
"LXTask 0.1.4" still shows Vivaldi processes running as root on my system but "top" and "ps -U root -u root u" both show Vivaldi running as user.
I"ve looked at the C code for the XFCE task manager (found at http://git.xfce.org/apps/xfce4-taskmanager/snapshot/xfce4-taskmanager-1.1.0.tar.bz2), specifically task-manager-linux.c, hoping to find code that I at least somewhat understood but, as my expertise is in IBM Assembler, I saw very little intelligible detail.
I agree with Gort that the task managers are likely at fault but I'll continue to use Opera 12.16 until the situation clears.
Thanks to all,