When you cannot make Selenium trigger events you rely on (e.g. a "change" event when filling in a form field), trigger it yourself using this step:
When /^I manually trigger a (".*?") event on (".*?")$/ do |event, selector|
page.execute_script("jQuery(#{selector}).trigger(#{event})")
end
Note that this only triggers events that were registered through jQuery. Events registered through CSS or the native Javascript registry will not trigger.