What "Edit comment box"? I tested ENTER when writing a comment, and it started a new line as expected.

It would be really bad if ENTER submitted a post instead of starting a new line. That would be totally unintuitive, and how would you then start new lines?

Olin Lathrop‭ Edited to clarify. This is when editing a post, not a comment. An "Edit Comment" is called an "Edit summary" on Wikipedia and Stack Exchange.

It's still not clear exactly what it is you are typing into that this question is about. However, it would be very unintuitive if ENTER caused a post to be submitted. ENTER is for starting a new line.

The only time ENTER might be expected to submit what was typed is when it's a short string limited to a single line. A good example would be entering a search string. You don't expect to write multiple lines, and you do expect ENTER to submit the search request in that case.

Olin Lathrop‭ "It's still not clear exactly what it is you are typing into that this question is about." - As I wrote, "when I edit a post, pressing Enter in the Edit Comment box" - what's unclear about that? In any case, tricho's answer has a screencap.

"when it's a short string limited to a single line." - Yes, that's the case here.

I don't know what an "Edit Comment box" is. What exactly triggers you to see this thing? When you edit a comment, you see and edit multiple lines. The answer with the animation shows something with multiple lines, and no context as to what you had to do to get there.

Olin Lathrop‭ again, 'This is when editing a post, not a comment. An "Edit Comment" is called an "Edit summary" on Wikipedia and Stack Exchange.' And by "post" I mean a question or answer.