Fix duplicate TextArea labels in Unity 6 inspectors

[mackysoft]

Summary

Fix duplicate TextArea labels when the TextArea is the last field drawn by SubclassSelector in Unity 6. Unity's IMGUI TextArea defers its prefix label during Layout; requesting a passive control ID after child drawing resolves that label inside its original IMGUI context.

Scope

• Apply the workaround at the shared boundary after expanded child drawing, covering normal children, nested fields, and custom drawers.
• Keep the change in the Editor drawer under UNITY_6000_0_OR_NEWER.

Verification

• Reproduced in the reported Unity 6000.3.12f1 on macOS using package 1.7.0 and pre-fix main (7c81c92). Compared TextArea last versus a following number field, standard inline fields, and managed references without SubclassSelector.
• Confirmed one label after the fix for lists, direct managed-reference fields, nested fields, and custom drawers. Entered eight lines, scrolled the TextArea, collapsed/reopened the field, and selected another type through the popup with the entered text retained.
• Independent static review: no actionable findings.
• Linux Unity CLI validation: Unity 2021.3.45f2 22/22 passed, Unity 2023.2.22f1 25/25 passed, final Verification passed. The Unity 6 visual regression was checked directly in the Editor.

References

• Unity TextAreaDrawer
• Unity deferred prefix-label handling

Closes #105