Usually the application do not have to have the focus for scrolling events. Just the mouse cursor has to be over the area I want to scroll. (Except Microsoft Excel 
)
Great find Boris. If you use ⌘+TAB to switch to Aspect and ⌘+TAB to switch back to another application than the background scrolling works. If you do this with a mouse/trackpad click it does not work.
This is clearly a bug and I will move it to the Bug Report section. Thanks!
Tracked under #443
Fixed in 1.0.0-preview.3