Based on the issue you're describing, the rendering problem with your character's eye (scaled to -1) in UI Toolkit with Screen Space Overlay is likely related to backface culling.
When you flip a bone via Scale X set to -1, front-facing triangles become backfacing ones. UI Toolkit unfortunately discards these backfacing triangles, and there doesn't appear to be a setting to customize the cull mode, as VisualElement does not allow setting a custom Material or Shader.
Solution for the rendering issue:
A simple workaround would be to change the root bone Scale X value from -1 to 1 in your Spine project and instead flip the UI visual element in Unity. This way, you avoid the backface culling issue entirely.
You can verify this is the cause by enabling Viewport - Backface Culling in the Spine Editor settings to see if the same parts disappear there.
Regarding custom shaders and missing features:
While Unity 6 now allows writing custom shaders for UI Toolkit, there are currently some limitations:
- In general, no custom shaders are used with UI Toolkit by default (you can't set a custom shader in earlier versions)
- This means it does not support PMA (Premultiplied Alpha) or advanced features yet
- The Spine URP Shaders and UITK support is only officially available on Unity 6 due to some API extensions missing in earlier versions
Regarding physics support in UI Toolkit:
I'm not aware of specific plans to add physics support to UI Toolkit at this time. Physics in UI contexts presents unique challenges, and you may need to implement custom solutions for your specific use case.
If you continue to experience issues after trying the scale workaround, please ensure you're using the latest spine-unity 4.2 version (4.2.100 or newer) for the best Unity 6 compatibility.
