How to reproduce:
1. Create a new Unity project
2. Go to Preferences -> Overlays -> Overlay Background -> Any color
3. In the Scene view, make sure the Overlay Menu is set to the "Panel"
4. Move the Overlay Menu to the left side of the Scene view and dock it
5. Expand and collapse the Overlay Menu and observe the results
Expected results: The custom color is displayed both in the collapsed and expanded states
Actual results: The custom color is displayed only in the expanded state which is not consistent
Reproducible with: 6000.3.0a5
Can't test with: 6000.0.56f1, 6000.1.16f1, 6000.2.2f1
Reproducible on: macOS Sequoia 15.5 (M1)
Not reproducible on: No other environment tested
Note: Reproducible with both themes identically