Selenium 2/WebDriver and Selenium Server - switching windows gives same HTML - selenium

Question

I've been using Selenium 2/WebDriver through Ruby 1.9.2, and directly through FireFox it works fine. I wanted to use HtmlUnit so it's faster and headless, so I'm trying selenium-server-standalone-2.17.0 as it should in theory require no code changes, and also I want to start using Java/groovy with HtmlUnit.
The problem is that half the time or more, a test fails because when I click a button and switch windows, the HTML (and title) in the new window is the same as the old one.
I added lots of debugging output to my function to try to narrow it down:
def switch_to_newest_window()
assert(#driver.window_handles.size > 1, "only one window")
print "switch to newest window, handles=#{#driver.window_handles}...\n"
print "current handle: #{#driver.window_handle}\n"
print "#{#driver.window_handles[0]}\n"
print "#{#driver.window_handles[-1]}\n"
print "title: #{#driver.title}\n"
save_file("first.html", #driver.page_source)
#driver.switch_to.window(#driver.window_handles[-1])
print "new handle: #{#driver.window_handle}\n"
print "new window title: #{#driver.title}\n"
save_file("second.html", #driver.page_source)
end
And the relevant output is:
switch to newest window, handles=["36543124", "1755893858"]...
current handle: 1755893858
36543124
1755893858
title: Create FlowSet
new handle: 1755893858
new window title: Create FlowSet
So it's correctly switching windows, but the HTML is the same! If you diff first.html and second.html, there is no output.
I'm also a ruby noob so I may well be doing things the hard/slow way or incorrectly.
Other details:
running on Windows 7.
Selenium Server server-standalone-2.17.0
Server output when starting HtmlUnit session:
13:03:12.292 INFO - Executing: [new session: {platform=ANY, javascriptEnabled=true, cssSelectorsEnabled=false, browserName=htmlunit, nativeEvents=false, rotatable=false, takesScreenshot=false, version=}] at URL: /session)
Thanks very much in advance for any help.
Joel