How to reproduce:
1. Open the user's attached to project "UIToolkit-PropertyField-RightClick.zip"
2. Select "GameObject" Prefab in the Project window
3. In the Inspector window find the "Monobehaviour IMGUI (Script)" component
4. Right-click on the "That Struct" header and notice that the context menu appeared
5. In the Inspector window find the "Monobehaviour UI Toolkit (Script)" component
6. Try right-click on the "That Struct" header

Expected result: The context menu appears and it behaves the same as IMGUI's context menu
Actual result: The context menu doesn't appear and its behave differs from IMGUI

Reproducible with: 2020.2.0a8 (built-in UI Toolkit), 2020.2.2f1 (built-in UI Toolkit, UI Toolkit 1.0.0-preview.13), 2021.1.0b4, 2021.2.0a3 (built-in UI Toolkit)
Not reproducible with: 2019.4.18f1, 2020.2.0a7 (built-in UI Toolkit)
Could not test with: 2018.4.31f1 (built-in UI Toolkit), 2020.2.0a7, 2020.2.0a8, 2021.1.0b3, 2021.2.0a2 (UI Toolkit 1.0.0-preview.13) (due to errors)

Notes:
- Up to 2020.2.0a7 version, built-in UI Toolkit's Serializable object's header behaves the same as IMGUI's Serializable object's header. Context menu doesn't appear after right-clicking on UI Toolkit's and IMGUI's Serializable object header