Chainsaw chain "stretch" is really wear.

The metal doesn’t stretch like a rubber band. The rivets and drive links wear down, making the chain longer. Same result: it doesn’t fit the bar right anymore.

Main causes: running the chain too loose, cutting with a dull chain (which heats things up and accelerates wear), and not enough bar oil. Heat is the enemy. A hot chain expands, then it contracts when it cools, but the wear stays.

How to tell it’s time to replace: If you tighten the chain and within a few cuts it’s sagging again, that’s the sign. Or if the chain rides up on the bar rails and you can’t get it to stay tight. Another clue: the cut goes crooked no matter how straight you hold the saw. That’s uneven wear.

Don’t keep tightening. Once the chain won’t hold tension, replace it. A new chain is cheaper than a new bar and sprocket.