A new long-term support release of Xubuntu is out, and I have upgraded to it on my desktop. Make sure to check out the Xubuntu release notes as well as the general Ubuntu release notes before upgrading yourself.
The new version was the smoothest upgrade I have ever had — taking less than an hour to do all of its work. There is a new document viewer — Atril — which has been great so far. And, the display bug I previously had with System Load Monitor (see my setup notes) has been fixed. With the old indicator and notifier plugins, the Dropbox display bug and a similar InSync display bug that started with version 1.3.20 of InSync and Xubuntu 17.10 remain.
After switching to the new PulseAudio, Status Notifier, and Notification plugins, the DropBox display bug is now eliminated.
As this pic indicates, however, the Status Notifier and PulseAudio plugins should have re-sizable icons. Right now, they are too large for the panel.
The major problem so far with Bionic Beaver is the loss of pdftk, a command-line program for combining, separating, and bates-stamping PDF files. There is a bug report about pdftk being removed from 18.04. At the moment, possible solutions include: downloading the older binaries and running dpkg to install them, installing a java-ized version or linking to a 16.04 docker-version (see this Q&A) or re-activating sources connected to the prior version of Xubuntu/Ubuntu to get access to pdftk from there.
There are new (to me) PDF command line tools — pdfseparate and pdfunite — for separating and combining PDF docs. But, pdftk does much more and is quite elegant in handling its tasks. I very much want pdftk back. For now, I downloaded the older binaries and ran dpkg to install them. So, I have pdftk back on a temporary basis at least.