Function placeChild

packages/react-reconciler/src/ReactChildFiber.js:492–520

(
    newFiber: Fiber,
    lastPlacedIndex: number,
    newIndex: number,
  )

  }

  function placeChild(
    newFiber: Fiber,
    lastPlacedIndex: number,
    newIndex: number,
  ): number {
    newFiber.index = newIndex;
    if (!shouldTrackSideEffects) {
      // During hydration, the useId algorithm needs to know which fibers are
      // part of a list of children (arrays, iterators).
      newFiber.flags |= Forked;
      return lastPlacedIndex;
    }
    const current = newFiber.alternate;
    if (current !== null) {
      const oldIndex = current.index;
      if (oldIndex < lastPlacedIndex) {
        // This is a move.
        newFiber.flags |= Placement | PlacementDEV;
        return lastPlacedIndex;
      } else {
        // This item can stay in place.
        return oldIndex;
      }
    } else {
      // This is an insertion.
      newFiber.flags |= Placement | PlacementDEV;
      return lastPlacedIndex;
    }
  }

  function placeSingleChild(newFiber: Fiber): Fiber {
    // This is simpler for the single child case. We only need to do a