Thin black or white line at the bottom of the screen

You perhaps describe the iPad’s Home Bar…

The Home Bar (the line seen at the bottom of the screen) is User Interface (UI) feature of iPad models that do not have a physical Home Button. The bar can be dragged left or right to access other App windows without first invoking the Task Switcher.

iPad models that do have a physical Home button do not have the Home Bar; this is not a configurable option.

Hey ami811,

We'd be happy to help out. If this is something new you're suddenly seeing on the display of your iPad Pro that's unexpected, it could indicate a hardware issue with the display. Without any photo showing specifically what you're seeing though it's difficult to say.

If the line continues to appear though even after restarting, it'd be best to have the device evaluated further by a technician at an Apple Store or Apple Authorized Service Provider in your area. For help checking the service options nearby, visit: Get Support

Take care!