WebEngine apparently caches the page that contains cache manifest based on network traffic, but when opening the page again after disconnecting from the internet, following happens:
Qt 5.5.1, error page is displayed with "Show saved copy" button. How ever, the button does nothing. Looking at the code, pressing the button seems to rely on window.errorPageController, which is not defined.
Qt 5.6.0-beta1, "Show saved copy" is hidden completely.
Can be tested at: http://vps122778.ovh.net/joni/offline