dillon1000/react

Root API should clear non-empty roots before mounting (#18730)

* Root API should clear non-empty roots before mounting

Legacy render-into-subtree API removes children from a container before rendering into it. The root API did not do this previously, but just left the children around in the document.

This commit adds a new FiberRoot flag to clear a container's contents before mounting. This is done during the commit phase, to avoid multiple, observable mutations.