First of all, this is not solution - it's a workaround.
I'm using Opera 56.0.3051.104 on Xubuntu 18.04 but i think you can apply this on any other version of Linux DE.
You have to modify opera's launcher:
Right click on it, Properties, Pencil icon (edit the currently selected item), Command field (third from above) where opera %U is already written. Overwrite with:
opera --window-position=118,64 --window-size=1682,951 %U
That's actual settings for my 24' display and the Opera window is now positioned on the center of the screen in windowed mode. Of course, you can play with numbers (X and Y) until you get the desired position.