I recently had to write a small capybara helper:

def without_page_refresh
  page.execute_script("window._withoutPageRefresh = 'BAM'")
  yield
  page.evaluate_script("window._withoutPageRefresh").should == "BAM", "Page was navigated away"
end

Use case:

You are building a form that gracefully degrades
when javascript is not enabled. It goes something like this:

# uses rack-test so no javascript
scenario "User can lose money in an accessible way" do
  visit "/your_account"
  page.should have_content "You have $100"
  click_button "Charge me"
  page.should have_content "You have $50"
end

Next step is to make that form do the ajax thingy.
You copy test above and switch it to use javascript driver:

# uses selenium for javascript
scenario "User can lose money with style", :js => true do
  visit "/your_account"
  page.should have_content "You have $100"
  click_button "Charge me"
  page.should have_content "You have $50"
end

Newly written test is pretty good except that it passes without
writing a single line of javascript. So here’s when that helper
comes into play. Our javascript test becomes:

# uses selenium for javascript
scenario "User can lose money with style", :js => true do
  visit "/your_account"

  without_page_refresh do
    page.should have_content "You have $100"
    click_button "Charge me"
    page.should have_content "You have $50"
  end
end

Now javascript test fails with “Page was navigated away”
since nothing is preventing that form from submiting to another page.