OK, I checked the code (way more accurate than empiric tests), and it is NOT a balance change that triggers the interest but any operation that involves HBD.

Any operation involving assets call function(s) (database::adjust_balance() which calls database::modify_balance() for liquid, or database::adjust_savings_balance() for savings) which will do the interest payment if required, whether the balances really change or not.

Even cancelling a withdrawal from savings can trigger an interest check and potential payment.

So it's better to tell people that any operation involving HBD could trigger interest payment!