Neither a long nor a short position is materially affected by a stock split - the value of the position does not change. So, if a company has announced that it will split in six months, it should have no bearing on the attractiveness of the short investment.
If a forward split, the shorter will owe more shares at reduced price. If a reverse split, the shorter will owe fewer shares at a higher price. So, no value change.
However, if the share price increases or drops AFTER the split, the shorter will be affected.
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