I personally do not like the fact that when I check emails, that bloody side panel always pops out. Annoying as it is, I simply immediately close it and continue on. I also had to customize the layout of the built-in mail reader, but that is not too difficult.
Have you tried the Opera - Settings function? I think it allows you to change the placement of panels and toolbars if I am not mistaken.
I guess I didn’t feel the impetus to do all that. Maybe I will at some point in the future.