![Screenshot 2019-12-16 10 51 05](https://user-images.githubusercontent.com/793565/70934095-fd91ed80-1ff1-11ea-93b5-746e816585ec.png)
There's seems to be circumstances where unmounted DOM/Fibers are kept alive by React DevTools. They're kept alive in `primaryFibers`:
https://github.com/facebook/react/blob/34527063083195558f98108cde10b5d6ad0d6865/packages/react-devtools-shared/src/backend/renderer.js#L772

It seems like a WeakSet would be appropriate and would remove the leak. Otherwise we'd need to understand why recordUnmount isn't called.

CC @bvaughn  
1
       T
       W
      unmountFiberChildrenRecursively(prevFiber);
       2
      const nextFiberChild = nextFiber.child;
      const nextFallbackChildSet = nextFiberChild
        ? nextFiberChild.sibling
        : null;
      if (nextFallbackChildSet != null) {
        mountFiberRecursively(
          nextFallbackChildSet,
          shouldIncludeInTree ? nextFiber : parentFiber,
          true,
          traceNearestHostComponentUpdate,
        );
        shouldResetChildren = true;
      }
    } else {
       C
       T
      if (nextFiber.child !== prevFiber.child) {
         I
         E
        let nextChild = nextFiber.child;
        let prevChildAtSameIndex = prevFiber.child;
        while (nextChild) {
           W
           t
           S