Fixes #2527
The issue at hand: when updating components with children who are strictly equal or implement SCU they could have reordered due to the parent rendering. This works when the children are just DOM-nodes but becomes an issue when these children are Fragments or nested Fragments. We need to update _nextDom so the parents diffChildren can know where to continue after it has diffed this child, so for instance when we see the following:



const Child = () => (
  <Fragment>
    <div>1</div>
    <div>2</div>
  </Fragment>
);

const Parent = () => (
  <div>
    <Child />
    <div />
  </div>
);



The _nextDom property should signal to diffChildren(Parent) that after the diff of children.Child it has to place the <div /> after the last dom-property wrapped by the Fragment in this case <div>2</div>, but what happens if our Child bails out of render, and returns props.children? These children could be reordered, … by the Parent component. This means that our dom-reference could be stale.

When we aren’t bailing out of render the placeChild function handles this by tracking _nextDom and reordering everything however when we bail out there is no such logic. For this we abstracted out the placeChild logic and reused it in SCU/strict-equality bailout, this deeply traverses the DOM when it sees that it isn’t equal and does the placement. This is still an optimized path as compared to non-scu/non-bailout.

I guess we could introduce better bailouts but up to now I'm not entirely sure how yet.