I have using Supersized background slideshow. and tried it on iPhone and iPad, you can see the slideshow on all the screen, but when you rotate the screen (so you browse the website horizontally) the slideshow won’t work on all the screen!! unless you refresh the page.
Also, if you refresh the page on horizontal view and rotate the screen to vertical view, you can see the slideshow perfectly but, if you rotate again to the horizontal view, the same issue will occur which is the slideshow won’t display on all the screen. Please see the attached screenshots ( one from an iPhone and another one from an iPad mini).