React DevTools might retain references to unmounted DOM elements (and their Fibers)

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

[bvaughn]

Quick update about this issue and PR #17666

The thing that caused the above temporarily “leak” is the alternate pointer- which will be cleared during the next render. That being said, the reason the temporary “leak” retains so much (the whole subtree) is because we weren’t nulling out the stateNode field. PR #17666 changes this behavior so that we now will null that field. This should make our memory snapshot comparisons less noisy in the future.

In other words, hopefully this will show that DevTools isn’t actually leaking DOM nodes (and Fibers) but if it is at least we should be able to identify them less ambiguously now.

[piecyk]

Thanks @bvaughn 👍