Why does a UI Toolkit-only drawer fail inside some Inspectors?

[mackysoft]

A subtype has a custom PropertyDrawer implemented with CreatePropertyGUI, but its fields do not appear in some Inspectors.

Which Inspector setups can host a UI Toolkit-only drawer, and what is needed to support IMGUI Inspectors as well?

[mackysoft]

SerializeReferenceExtensions 2.0.0 supports both IMGUI and UI Toolkit. A selected subtype's custom drawer still needs an implementation that its parent Inspector can use.

Unity cannot run a UI Toolkit PropertyDrawer inside an IMGUI custom Inspector or a parent IMGUI drawer. A drawer that only implements CreatePropertyGUI therefore needs a UI Toolkit parent. To support IMGUI parents too, provide OnGUI and the corresponding GetPropertyHeight implementation.

In Unity 2021.3, use a custom UI Toolkit Inspector to host UI Toolkit-only drawers. From Unity 2022.2, the default Inspector uses UI Toolkit for custom property drawers, while custom Inspectors can still use IMGUI.

Unity's PropertyDrawer documentation includes examples of both implementations.