This breaks the properties thing as soon as the flatten-exception is unserialized, isn't it?

Can't we use __serialize/__unserialize instead? This might break child classes of FlattenException because __sleep doesn't work with them.

I think a better course of action could have been to put a VarDumper's Data object instead of what is currently done, or to convey the properties outside of the FlattenException is possible? Maybe via a child class actually?

If we use a Data object, the serialized payload can potentially be huge (as is the case after #49620, ignoring unserializable values.)

/cc @lyrixx for thoughts

I gave my comments a try and submitted #49746 instead. Thanks for giving this a try!