A single-line text box that has no possibility of multi-line text (so, not a chat interface), such as search, an address bar, something that's obviously "submit one item" (e.g. "submit a word"), etc.

In a multiline text box, enter should NOT submit the form. Chat interfaces violate this rule and it results in lots of premature chat submissions.

Precisely. 'member CUA?

In single-text-input contexts, like search fields and the browser address field, and things like Save As dialogs. It’s the general expectation for dialogs with an OK or default button, just like Escape cancels the dialog.

A search box, I think

The new idiom:

You are right, of course this is your account name! Do you want me to be keep you logged-in?

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