showDocument() does block - are you
saying it is blocking other threads as well? There
are some well-know bugs under Solaris in certain
versions of the JVM that cause I/O operations to
block other threads, but the Bug Database has
no mention of a problem with showDocument().

If you believe that what you are seeing is improper
behavior, report it to Sun using the Bug Database
at http://java.sun.com/.
You will have to provide a much more complete description
of the problem than you posted here if you want them to
do anything - include sample code that reproduces the
problem and don't forget to specify which OS, which
JVM (and what version), which browser, etc., under which
you saw the problem.