I mean, this is fine as it goes, but is there really a big benefit to having an envelope corresponding to an imaginary straight arrow when the shaft is not straight?

No the envelope does not correspond to a straight arrow, it's the actual arrow with a point head and tail

Oh, I see, I misunderstood. In that case, this is essentially equivalent to what the behavior used to be. The resulting envelope may be slightly wrong (potentially resulting in part of an arrow getting cut off at the edge of a diagram, etc.) since the actual position of the shaft depends on the head & tail size. We got rid of this, the reasoning being that it's much easier to explain that arrows have no envelope than it is to explain that diagrams have envelopes that may be slightly wrong.