I think that these are idioms, but idioms which are easy to understand, so not "very idiomatic". "When I saw how disappointed she seemed to feel" or "When I saw her disappointed expression" would not be idioms.
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Grammar GeekI don't think it's an idiom at all, but literal.Thanks, GG.
I'm not sure how her being disappointed can be fixed by accepting her gift or what that has to do with thanking her, but the part about being able to see the disapoinment in her eyes is perfectly natural!