![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  
}
     Also make sure we're tracking its alternate,
     e.g. in case this is the first update after mount.
    const {alternate} = fiber;
    if (alternate !== null) {
      if (!fiberToIDMap.has(alternate)) {
        fiberToIDMap.set(alternate, refinedID);
      }
    }
    if (__DEBUG__) {
      if (didGenerateID) {
        debug(
          'getOrGenerateFiberID()',
          fiber,
          fiber.return,
          'Generated a new UID',
        );
      }
    }
    return refinedID;
  }
   Returns an ID if one has already been generated for the Fiber or throws.
  function getFiberIDThrows(fiber: Fiber): number {
    const maybeID = getFiberIDUnsafe(fiber);
    if (maybeID !== null) {
      return maybeID;
    }
    throw Error(
      `Could not find ID for Fiber "${getDisplayNameForFiber(fiber) || ''}"`,
    );