When is a a comma needed?

Below are two variations of a sentence. Sentence 1 is clearly a compound sentence with two independent clauses, and the accepted rule is to put a comma before the conjunction (as here). Sentence two appears to be a simple sentence with a compound predicate. Some editors advise putting a comma before the "and" that connects the second predicate. Is that correct? What is the reasoning behind the inclusion of a comma there?

  1. On warm summer days, I swim in the cool ocean, and I take long walks on the shadowy forest trails.

  2. On warm summer days, I swim in the cool ocean and take long walks on the shadowy forest trails.


Solution 1:

A compound predicate with [exactly] two verbs should not be separated by a comma (OWL [13], Grammarly).

The one exception would be where there is a chance of mistaking the meaning ("I smiled at the man who walked in and waved." Did I wave, or did the man?).

I could not find any examples of editors or style guides recommending the inclusion of a comma, but doing so would be grammatically incorrect.