Hi perknh,
I found the issue: it's related to nvidia proprietary drivers! (I suspected)
Solved by using nouveau drivers, but it is just a workaround.
I'm a web developer! I use Opera to develop all of my sites and applications.
I think this issue affects many people: all of nvidia drivers I try give the same result: weird rendering.
I wonder if there's a way to better inspect this problem.
Finally, I'm in love with Opera since 2006 so I'd like to help solve this. It's a pity that many people could just switch to Chrome to solve this issue..

Hi, I'm experiencing a weird rendering problem starting from Opera 35 (if I remember well).
Looks like the in some elements Opera adds a pixel and breaks the layout.
I cannot find the way to attach some screenshot I've done.
The problem is visible looking to this site:
https://www.latruccheria.it
The social buttons aren't rendered properly.
On this page:
https://www.latruccheria.it/prodotti
The sidebar jumps to the bottom of the layout.
I use Linux Mint 17 with a nvidia video card and proprietary drivers.
The problem is still present on Opera 38 and 39.