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do overly long or very short stems affect the handling?
Yes and no.
It affect how it feel, but doesn't affect how it ride.
Bit hard to explain, but put it this way, putting a 120mm stem on a bike may make it feel slower, but doesn't mean it'll actually be slower to turn in corner compare to a 90mm stem.
Human are very adaptable, once you get used to the short stem, it feel normal, ditto to long stem.
The whole point of the stem beside holding the handlebar is to fine tune your fit.