![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  
flushPendingErrorsAndWarningsAfterDelayTimeoutID = null;
    }
  }
  function flushPendingErrorsAndWarningsAfterDelay() {
    clearPendingErrorsAndWarningsAfterDelay();
    flushPendingErrorsAndWarningsAfterDelayTimeoutID = setTimeout(() => {
      flushPendingErrorsAndWarningsAfterDelayTimeoutID = null;
      if (pendingOperations.length > 0) {
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        return;
      }
      recordPendingErrorsAndWarnings();
      if (pendingOperations.length === 0) {
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        return;
      }
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      const operations: OperationsArray = new Array(
        3 + pendingOperations.length,
      );
      operations[0] = rendererID;
      operations[1] = currentRootID;
      operations[2] = 0;  S
      for (let j = 0; j < pendingOperations.length; j++) {
        operations[3 + j] = pendingOperations[j];
      }
      flushOrQueueOperations(operations);