HelpProvider's popup blocking thread

We have a messaging application that makes seemingly unremarkable use
of a HelpProvider component. However, after we've received a message,
if we display the help popup (either via F1 or through the Help menu)
and attempt to search in the search tab, no results display, the
application quits listening to mouse events, but any dialogs continue
to repaint themselves.

Task manager thinks our application is still responding, but trying
to close it then brings the "kill now or cancel to wait" dialog.

The problem appears to be some hangup in the event loop because if we
receive another message, upon which we display a notification dialog,
the help popup wakes up and shows the search results and mouse events
are again interesting to the app. Following execution in the debugger,
the wakeup happens immediately after control returns from the call to
ShowDialog.

Has anyone seen behavior like this before? Any ideas on other
directions to look?

Followups set to microsoft.public.dotnet.general.

Thanks,
Greg