I should have looked more carefully before wasting everyone's time with the post below.  In Tools->Preferences->Viewer/Editor there is a checkbox labeled "Use Classic Preview Pane", and that fixes the problem :-)

I just got an upgrade license for v10 to v11 (.4.2.1) and am just starting to try it out.  I've looked through some of the discussions about restoring the pre-v11 appearance of various things, and I've seen the list of recommended items to change.  I've probably just overlooked it, but I haven't been able to find anything on how to make the message view pane (if that's even the correct term for it) look like it did in previous versions.  What I am calling the message view pane is the pane below the message list where the content and details of the selected message are shown.  It used to be nice and compact, but now it reminds me of all the wasted space on a webmail page.  Is there any way to switch back to the pre-v11 view without switching back to a pre-v11 version of TB?