>> I think before pixel-level vertical scrolling up or down,
>> scrolling single line up or down `with pixel transition' should be
>> implemented.  It ends up (window-vscroll nil t) to be zero.
>> This may be good enough to give modern feel.
>> 
>> I assign the snippet to mwheel-scroll-up-function and it works
>> OK so far.
> 
> Sorry, I don't understand your proposal.  Can you show some code?

Yes.  I attach a package in a middle.  With this Emacs scrolls a line
with pixel transition but ONLY upward.