Chromium and thereby Opera impose on Linux, Windows-like compatibility with scrollbar "snap back" when moving the mouse away from the slider it will jump the page slider+content back to the drag origin.
Back when introduced in Chrome about may 2014, I tried getting used to this for at least one month without success, maybe part of the failure to adapt was my high DPI mouse - seemingly at random every single time I read a long page, the content would jump to the beginning of the page/slider origin forcing me to lose focus and push the cursor to the right before getting the content back.
I don't know how Windows users live with this - I think they may simply have given up using the mouse cursor for scrolling long pages.
So I go between Firefox (which slider behave like all the other native apps which is lovely) and Chromium+snapback_removal_patch when I feel Firefox is just too slow at loading sites..
I compile Chromium with a patch I found here: google product forums.
..Compiling Chromium every now and then is something I certainly could do without + seeing all the other fixes in Opera like how well it looks on my 4k monitor at work compared to Chromium - I thought - why not propose the patch to Opera team and hope in the future I can just go that way.
Though not by default, QT can do snapback if configured to use the Windows style, so for those who desire this - maybe let Opera follow this same configuration or use an option/env/flag?
You win the hearts of many Linux users by reverting or making configurable, this criticized "feature" which was decided upon seemingly solely by one Chromium dev.
Further background on the subject can be read in the same forum where I found the patch for Chromium.
Even more background near the end of this post: http://wildlava.tumblr.com/post/93132151021/google-chrome-now-imposes-windows-scrollbar .