This is great question. There is clash of ideas here. When a conjunction tags on an independent clause (i.e., what could be standalone sentence), then it is preceded by a comma.

I like pies, and I like cakes.


That's clear. The confusion arises when an independent clause feels like a list item of a two-item list.


"He flicked the match out the open window and I heard it hiss in the water."

This is two independent clauses, and a comma would be expected. However, a comma would ruin the flow of text. So, lots of people would omit it. (Quite acceptable in my opinion.)

It's related to the idea of listing full sentences.

I came, I saw, I conquered.

That's technically a run-on sentence, but it's acceptable. The bottom line is this: The "rules" do not address using independent clauses in lists very well.

So, you are safe to let instinct guide you.

Thank you so much for your reply. It was clear and helpful. :)