@chienetfou
I don't have an icon on the desktop. I have a panel widget which would qualify as a menu entry.
I am using EthanNisimura's fix.
Each Opera update does reset /usr/share/applications/opera.desktop to Exec=opera %U but that doesn't seem to be an issue. I've been leaving it unchanged recently.
What seems to matter is the /.local/share/applications.opera.desktop file. It doesn't get changed with updates.
As long as it remains Exec=opera --window-size=1920,1080 then Opera always stays maximized.