As I understand it, in a situation where two miners solve a block at the same time, a branch is created on the blockchain which is resolved when one branch is built upon, and thus becomes longer than the other branch. The longer blockchain is then regarded as the de facto blockchain.
But what happens to the confirmed transactions in the shorter branch? If this shorter branch is now disregarded, are the transactions lost? If so, are bitcoins lost as a result? Are the transaction sent back to the miners' mempool? If so, is there a risk of double-spending?