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If a firewall is present and the just-installed .msi file is followed by
  a use of the installed java command (my) firewall GUI pops up and asks permission to let
  the program continue. If I ignore this for a "long time" (don't know
  for sure, probably several minutes in this case), then when I do let
  it proceed the BackgroundDownload process seems to hang with a .tmp
  file (nio bundle, I think) in the bundles directory. If I remove the
  .tmp file, kill and restart the program again the background install works
  OK. I think there is a network timeout happening and control is
  returning to DownloadManager Java code rather than throwing an exception
  and surprising it about the expected state of the download in question. But that's
  a theory.