Features
- Add
Field.set_defaultto change a state var's default, given exactly one ofdefaultordefault_factory, such asState.__fields__["items"].set_default(["a"]). A mutabledefaultis copied when set and again for each instance, so sessions never share it. TheTypeErrorfor assigning a state var through its class now points atset_defaulton the field of the state that declares the var. (#7519)