![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  
W
          if (nextChild.alternate) {
            const prevChild = nextChild.alternate;
            if (
              updateFiberRecursively(
                nextChild,
                prevChild,
                shouldIncludeInTree ? nextFiber : parentFiber,
                traceNearestHostComponentUpdate,
              )
            ) {
               I
               c
               p
              shouldResetChildren = true;
            }
             H
             t
             i
            if (prevChild !== prevChildAtSameIndex) {
              shouldResetChildren = true;
            }
          } else {
            mountFiberRecursively(
              nextChild,
              shouldIncludeInTree ? nextFiber : parentFiber,
              false,
              traceNearestHostComponentUpdate,
            );
            shouldResetChildren = true;