The Xiaomi MIX Flip is designed with a focus on minimalism and maximizing screen space, which is why navigation buttons are not included on the outer screen. The Xiaomi MIX Flip's outer screen supports only full-screen gestures.
The outer screen is intended for quick interactions, notifications, or viewing content at a glance, rather than extensive navigation, so gestures simplify and streamline the using experience without cluttering the display with buttons.
This design aligns with modern smartphone trends. Additionally, removing physical buttons allows for a more seamless design and potentially improves the durability of the outer screen.