Only detach nodes if necessary

[Rich-Harris]

Right now, Svelte detaches every child of every node during teardown. It only needs to detach the top-level ones.

[pillaiindu]

Is it fixed now?

[Rich-Harris]

yes! It's been fixed for over a year

[pillaiindu]

The performance of Svelte for swapping rows in this benchmark was very slow. How would it behave now?

[Rich-Harris]

The same. There's an issue for it: #588. It hasn't been a big priority because it's a fairly contrived example you don't often encounter in the wild