I'm a teacher and user of Google Classroom. When I am commenting to a student, return will take me to a new line, which is fine and useful. I cannot find, however, any information on what triggers the "post" instruction when I want to end the comment and send it. And the post command moves down the screen. If anyone is aware of a way to shortcut this, I sure would like to know. Searching for this on the web doesn't help either. It doesn't sound like anyone cares really.
On a Mac, in Safari in Catalina (But I'd be surprised if that would make the difference)