I would (naively as a regular user) expect one of two things to happen:
(a) the dash background to use the selected wallpaper; or
(b) [at least] the wallpaper to be visible behind the window spread.
Neither happens so any reasonable user is left confused. Also maybe annoyed that they don't get to see their favourite wallpaper photo while moving through the dash.
I would (naively as a regular user) expect one of two things to happen:
(a) the dash background to use the selected wallpaper; or
(b) [at least] the wallpaper to be visible behind the window spread.
Neither happens so any reasonable user is left confused. Also maybe annoyed that they don't get to see their favourite wallpaper photo while moving through the dash.