![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  
(The "_debugNeedsRemount" flag won't necessarily be set.)
    
     The best we can do is to delay untracking by a small amount,
     and give React time to process the Fast Refresh delay.
    untrackFibersSet.add(fiber);
    if (untrackFibersTimeoutID === null) {
      untrackFibersTimeoutID = setTimeout(untrackFibers, 1000);
    }
  }
  const untrackFibersSet: Set<Fiber> = new Set();
  let untrackFibersTimeoutID: TimeoutID | null = null;
  function untrackFibers() {
    if (untrackFibersTimeoutID !== null) {
      clearTimeout(untrackFibersTimeoutID);
      untrackFibersTimeoutID = null;
    }
    untrackFibersSet.forEach(fiber => {
      const fiberID = getFiberIDUnsafe(fiber);
      if (fiberID !== null) {
        idToArbitraryFiberMap.delete(fiberID);
         A
        clearErrorsForFiberID(fiberID);
        clearWarningsForFiberID(fiberID);
      }
      fiberToIDMap.delete(fiber);
      const {alternate} = fiber;
      if (alternate !== null) {
        fiberToIDMap.delete(alternate);
      }
      if (forceErrorForFiberIDs.has(fiberID)) {