PR# 16326 Zone gets into state where I cannot undock it

Although I cannot reproduce it, I have seen numerous cases when playing
with 6.5.80945 where a zone (or tool within that zone) cannot be undocked.
If I dock, undock and move things around enough, I always seem to get
into this state.  Note that this is on a rather slow machine (SunBlade 1000),
which might be relevant.

Sometimes, clicking on the title bar of another tool and trying to undock it
will "free up" the tool that cannot be undocked.  Other times, I have to
close the "frozen" tool and then bring it back via the View/Tools menu.