Thanks for your suggestion.
Due to limited resources recently we have no plan to add that feature.
I have accomplished Merlin's request by using Menucool ddmenu. I am using its responsive version, and change the @media query to switch to mobile mode by a quite larger width value(I set the switching threshold to be 1024px other than the 320px mobile width), so the menu becomes the vertical when the screen size is no larger than 1024px, and it works quite good for me.
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