Dragging horizontally with mouse

Occasionally I prefere to use the mouse instead of the keys when ajusting spacing in Engrave mode. Vertically I can choose that freely, but t seems like I cannot use the mouse for horizonal work. Is there some good reason for this?

The reason you can’t use the mouse for adjusting the note spacing is mostly technical. When you move the staves vertically with the mouse, you’ll notice that the music itself doesn’t appear, and instead you get a proxy for the staves you’re moving drawn as an overlay, with the music updated when you complete the drag. This is just about OK for adjusting vertical spacing, but wouldn’t be OK for horizontal spacing, and because Dorico would need to completely respace at least the system you’re editing and potentially other following systems when you drag the mouse in order to keep the display up-to-date, we couldn’t guarantee good enough redraw performance, so we opted to use the keyboard only. Hopefully in the future we’ll be able to extend this to using the mouse as well, but it’s not something that’s likely to happen imminently.