Give your iframe a name attribute (i.e. <iframe name="myframe">) and then simply use

When I press "Foo" in the iframe "myframe"
Then I should see "Bar!" in the iframe "myframe"

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Step as follows:

Then /^(.*) in the iframe "([^\"]+)"$/ do |step, iframe_name|
  browser = page.driver.browser
  browser.switch_to.frame(iframe_name)
  step(step)
  browser.switch_to.default_content
end