You can put me down as another Brit who has never heard this “idiom”, and I lived in Yorkshire (where ACGAS is set, I believe) for many years.
As you probably know, well as being a dessert, a “trifle” can also mean anything unimportant, so perhaps she dropped something unimportant.
(Biscuits or cookies are not trifles, except inasmuch as they are unimportant.)