Is screen sharing under Sierra secure, i.e., a fully encrypted connection? I'm trying to figure out if the screen sharing data is encrypted when I connect to my Mac at home from another Mac at work. I've seen a lot of questions and answers but I can't really find a definitive answer. Basically, I just have my Mac sitting at home with Screen Sharing turned on and I'm port-forwarding 5900. It works but again, I'd like it to be secure. 

Now, before anyone mentions SSH tunneling, I had it set up the first day I tried this (Remote Login, port-forwarded 22, tunnel created, etc) but then it stopped working. I'm still trying to figure out what's going on there but if Screen Sharing is completely encrypted, then I'm not going to worry about the tunnel anymore.
The answer depends on how you connect to it. 

If you're using a standard VNC client to connect with a password only, nothing is encrypted. Not even the password.

If you're using the Screen Sharing app on macOS 10.8 (Mountain Lion) or later to connect with a username/password or AppleID, everything is encrypted.

If you're using the Screen Sharing app on macOS 10.7 (Lion) or earlier to connect with a username/password, the exchange of credentials as well as keypresses and mouse moves are encrypted by default. However the actual screen display (i.e. the graphical content) is not. You can enable full encryption from the Preferences menu in the Screen Sharing app.

If you're using VNC, I would recommend wrapping it in encryption using for example an SSH tunnel, encrypted VPN or similar measures.