I just discovered the mistake. It has to do with reducing the size of the screen (top right rectangle). Matter thus settled.

@panosv said in No smooth scrolling in Opera?:

sapsalis.gr

Thank you for posting that website; the scrolling issue can be reliably reproduced on that page. The problem is not solved, and still exists in Opera 56. I feel that on some sites that I was visiting, it may have improved slightly, but overall, scrolling is still jerky and unresponsive in Opera as of the latest version of O56. Please visit the posted site sapsalis.gr (Greek meme site or something) and attempt scrolling, pinching-to-zoom, and diagonal and horizontal scrolling using a touchpad on a Windows laptop to reliably reproduce the problem.

@alx6 See and follow what's in https://forums.opera.com/topic/23068/instructions-on-posting-about-problems-in-opera-for-computers

I have nothing to suggest as a solution, but I have to mention something I noticed.
That scrollbar in the middle seems to be opera's native scrollbar (the grey one), while the one on the right seems to be the site's skinned scrollbar (the blue one). On the other hand, opening your site on my pc, shows only the native scollbar and on the right side.
One thing I can imagine is hardware acceleration? Do you have hardware acceleration active? And what gpu are you on?

I am on debian testing x64, with an old 9500gt, running the nvidia 340 legacy driver and with hardware acceleration enabled.