In other words, I am looking for something that is like pixel-scroll-mode in emacs.

As @Ekopalypse already pointed out, that feature doesn’t currently exist.

It might work to follow the directions in the FAQ for requesting a new feature. But there’s a good chance that it’s actually a scintilla feature, rather than a Notepad++ feature; in that case, you’d first have to find out whether that’s already implemented in a newer Scintilla version (Notepad++ uses Scintilla v3.56, whereas Scintilla itself is at 4.1.4 – though rumor has it the developer is working on upgrading to a newer copy of Scintilla in Notepad++).

If that feature already exists in Scintilla, then if/when newer Scintilla is integrated in Notepad++, the feature will be added to Notepad++.

If that feature does not yet exist in Scintilla, then add a Scintilla feature request, wait for it to be implemented there, then request Notepad++ to upgrade to that specific Scintilla version (ie, use that version when it next upgrades Scintilla).

Smaller steps of movement would result in a smooth scrolling experience, which would be desirable in a reading situation, where your eyes try to follow the text and would get distracted by the jerky movement of the viewed content.

In other words, I am looking for something that is like pixel-scroll-mode in emacs.

what would be the point of scrolling half a line or a quarter of a line ?

afaik - no.