If a block has a sole component child, we apply an optimization to not need a template. But if that sole child is wrapped in `$.async`, it can happens that the block's effect is assigned the wrong end node.

In the reproduction this happens because we have two components inside a Child which is rendered only after an async block resolves. In `$.append` we have logic to not reassign nodes when the active effect didn't already run, which it has by the time we come across it (because the async work had to resolve first; we added this in #17120 to prevent the opposite, nodes being "rewinded" to an earlier position) and so the second sibling component's `$.append` will not set its end node to the effect. As a result the end node is wrong (too early).

To fix this we assign the nodes in `$.async`. We could also use `compareDocumentPosition` in `$.append` to only ever go forward, but that feels a bit heavier performance-wise.

If a block has a sole component child, we apply an optimization to not
need a template. But if that sole child is wrapped in `$.async`, it can
happens that the block's effect is assigned the wrong end node.

In the reproduction this happens because we have two components inside a
Child which is rendered only after an async block resolves. In
`$.append` we have logic to not reassign nodes when the active effect
didn't already run, which it has by the time we come across it (because
the async work had to resolve first; we added this in
#17120 to prevent the opposite,
nodes being "rewinded" to an earlier position) and so the second sibling
component's `$.append` will not set its end node to the effect. As a
result the end node is wrong (too early).

To fix this we assign the nodes in `$.async`. We could also use
`compareDocumentPosition` in `$.append` to only ever go forward, but
that feels a bit heavier performance-wise.

---------

Co-authored-by: paoloricciuti <ricciutipaolo@gmail.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>