We have changed the shape (and the runtime) of React Elements. To help avoid precompiled or inlined JSX having subtle breakages or deopting hidden classes, I renamed the symbol so that we can early error if private implementation details are used or mismatching versions are used. The fix is usually to stop inlining the JSX runtime e.g. by configuring the bundler to treat react/jsx-runtime and react/jsx-dev-runtime as externals.

This also catches using a version of react-is that's not the same as react. react-is must be the same version as react.

Why "transitional"? Well, because this is not the last time we'll change the shape. This is just a stepping stone to removing the ref field on the elements in the next version so we'll likely have to do it again.