computed when the calculation is added to the store and don't change afterwards.
Otherwise, if their heights change when scrolling (due to a modification of the
display output type - ExactOnly, ApproximateOnly...), it generates crashes.
There was a problem with the way of computing Height, see
Calculation::HistoryViewCell. We did not encounter a problemeatic
scenario here, but there is no reason it would have been safe before.
