I have not had a score with a large number of staves, since the new mixer came out, till now. I could not find a way to scroll the mixer and resorted to extending it to the right and dragging the mixer to the left, far off screen.
I eventually decided to try hiding the taskbar and this revealed the scrollbar. I suppose changing the screen resolution might fix the problem. I have not tried it, as I do not wish to change that.
Surely it would be possible to reduce the depth of the dialog by 1.00 cm. to resolve this problem.