![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  
while (fiber !== null) {
       G
      getOrGenerateFiberID(fiber);
      if (__DEBUG__) {
        debug('mountFiberRecursively()', fiber, parentFiber);
      }
       I
       A
      const mightSiblingsBeOnTrackedPath = updateTrackedPathStateBeforeMount(
        fiber,
      );
      const shouldIncludeInTree = !shouldFilterFiber(fiber);
      if (shouldIncludeInTree) {
        recordMount(fiber, parentFiber);
      }
      if (traceUpdatesEnabled) {
        if (traceNearestHostComponentUpdate) {
          const elementType = getElementTypeForFiber(fiber);
           I
          if (elementType === ElementTypeHostComponent) {
            traceUpdatesForNodes.add(fiber.stateNode);
            traceNearestHostComponentUpdate = false;
          }
        }
         W
         b
      }
      const isSuspense = fiber.tag === ReactTypeOfWork.SuspenseComponent;
      if (isSuspense) {
        const isTimedOut = fiber.memoizedState !== null;