Vertical slider

I still find the vertical slider on the right side of the spectrum difficult to use. Especially, it is difficult to adjust the length of the slider.
It does fold out, but it also disappears again immediately. Then again, it is sometimes possible to move the spectrum vertically without the slider being visible. On my screen it is just 2.5mm wide.

Indeed you don’t need the slider to move the spectrum vertically : just grab the frequency scale and move the mouse up or down. You can also zoom in/out that frequency scale using the mouse wheel.
The vertical slider on the side is just an extra bonus, but if enough people use it I might add an option to make it permanently visible.

+1 for permanently visible as an option.