According to the Dutch resource Letsgodigital company
OPPO has patented two design options for smartphones with a slide mechanism. Both - with an additional screen.
Such a picture is quite suitable as an April Fools' rally, but the publication date is different from the day of universal fooling.
If you still accept the new design as a seriousdevelopment, you can try to speculate that this method of expanding the screen area has the right to exist on a par with folding screens - simply because of lower cost. And how convenient it will be to use this gift - the question is separate, it is not considered in the patent application.