Opera 44.0.2510.1449 on Ubuntu 16.04 Xenial.
Opera acts differently in different ubuntu distros, meaning in one right click on the title bar opens up a menu that has un-decorate, whereas another install doesn't have that. No fall down menu. I have no idea how that happens.
The menu is from your window manager, meaning it will be different depending on which desktop you are using. KDE (what I use) has a nice menu there, but I don't really know what Gnome, MATE, XFCE, LXDE and so on do.
I suppose you are right. In KDE (I have XFCE with Kwin) in Aliya Linux, that is based on Debian Stretch and in KDE Neon (Ubuntu), the menu is quite nice and I can have no border. In Ubuntu with Openbox (Nelum Openbox), also I can un-decorate. In Devuan (Nelum-Dev-1 Testing), it can also un-decorate. If the Ubuntu based distro has Gala Window Manager, it can't. Once there is no border, the minimize and maximize buttons go away. This doesn't happen in Windows 10. Maybe, the Opera devs take heed. I'm now using Opera Beta, for in a few days, it'd become 45 final.