В Capybara вкладки обрабатываются как новые windows, поэтому вам нужно перейти в новое окно, используя api окна Capybaras. Это будет выглядеть примерно так:
new_window = window_opened_by do
# here you are in the context of the original window but your actions are expected to open a new tab/window
click_link 'the link that opens the new tab'
end
within_window(new_window) do
# this is in the context of the newly opened tab/window
click_link 'the link inside the new tab'
end
# here you will be back in the context of the first window
new_window.close # if you want to close the tab that was opened