![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  
const maybeID = getFiberIDUnsafe(fiber) || '<no id>';
      const parentDisplayName = parentFiber
        ? parentFiber.tag +
          ':' +
          (getDisplayNameForFiber(parentFiber) || 'null')
        : '';
      const maybeParentID = parentFiber
        ? getFiberIDUnsafe(parentFiber) || '<no-id>'
        : '';
      console.groupCollapsed(
        `[renderer] %c${name} %c${displayName} (${maybeID}) %c${
          parentFiber ? `${parentDisplayName} (${maybeParentID})` : ''
        } %c${extraString}`,
        'color: red; font-weight: bold;',
        'color: blue;',
        'color: purple;',
        'color: black;',
      );
      console.log(
        new Error().stack
          .split('\n')
          .slice(1)
          .join('\n'),
      );
      console.groupEnd();
    }
  };
  
  const hideElementsWithDisplayNames: Set<RegExp> = new Set();
  const hideElementsWithPaths: Set<RegExp> = new Set();