![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  
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          const metadata = ((currentCommitProfilingMetadata: any): CommitProfilingData);
          metadata.durations.push(id, actualDuration, selfDuration);
          metadata.maxActualDuration = Math.max(
            metadata.maxActualDuration,
            actualDuration,
          );
          if (recordChangeDescriptions) {
            const changeDescription = getChangeDescription(alternate, fiber);
            if (changeDescription !== null) {
              if (metadata.changeDescriptions !== null) {
                metadata.changeDescriptions.set(id, changeDescription);
              }
            }
            updateContextsForFiber(fiber);
          }
        }
      }
    }
  }
  function recordResetChildren(fiber: Fiber, childSet: Fiber) {
    if (__DEBUG__) {
      debug('recordResetChildren()', childSet, fiber);
    }
     T
     T
     T
    const nextChildren: Array<number> = [];
     This is a naive implementation that shallowly recourses children.