<< only the background appears >>
then it is working as designed. The additional display is displaying a portion of your new Extended Desktop. The displays are concatenated along an edge (of your choosing) and used TOGETHER as if they were one larger extended desktop:
Once you have dragged the blue Icons to the same orientation as the physical displays on your bench, the mouse moves freely across the boundary between displays, and can carry a window across as well.
if you leave the window split between both displays, it will scroll "the right way"..