Under what circumstances are these two prepositions used in a grammatically precise manner? I posed this question because a grammar-editing application marked the indiscriminate interchangeable usage of these phrases as being grammatically correct.
I would imagine that these following examples are correct:
"I'm envious of how flawlessly you played the piano."
"I'm envious at the mere thought of your flawless playing."
However, the aforementioned application (Grammarly) found the sentence "I'm envious at how flawlessly you played the piano," to be faultless.
So, which of the above usages is correct?
(I've also searched the site for this question and I have not seen any duplicates, despite there being similar questions.)