![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  
Capture initial values as of the time profiling starts.
     It's important we snapshot both the durations and the id-to-root map,
     since either of these may change during the profiling session
     (e.g. when a fiber is re-rendered or when a fiber gets removed).
    displayNamesByRootID = new Map();
    initialTreeBaseDurationsMap = new Map(idToTreeBaseDurationMap);
    initialIDToRootMap = new Map(idToRootMap);
    idToContextsMap = new Map();
    hook.getFiberRoots(rendererID).forEach(root => {
      const rootID = getFiberIDThrows(root.current);
      ((displayNamesByRootID: any): DisplayNamesByRootID).set(
        rootID,
        getDisplayNameForRoot(root.current),
      );
      if (shouldRecordChangeDescriptions) {
         R
         F
         s
        crawlToInitializeContextsMap(root.current);
      }
    });
    isProfiling = true;
    profilingStartTime = getCurrentTime();
    rootToCommitProfilingMetadataMap = new Map();
  }
  function stopProfiling() {
    isProfiling = false;
    recordChangeDescriptions = false;
  }
   Automatically start profiling so that we don't miss timing info from initial "mount".