Background Size "Height" property becomes locked after undoing Scale Mode changes in UI Builder

Steps to reproduce:

1. Create a new project using Universal 3D template
2. Go to Assets > Create > UI Toolkit > UI Document (UXML)
3. Open the created UXML and add a Visual Element to the Hierarchy
4. Select the Visual Element and in the Inspector find the "Background" foldout
5. Set the "Position-X" and "Position Y" properties to the very left buttons
6. Set the "Scale Mode" to the very right button (scale-to-fit)
7. Click CTRL + Z a few times
8. Observe the "Height" property

Actual results: After the Undo operation, the Size "Height" property becomes disabled/greyed out in the Inspector

Expected results: The Size "Height" property remains enabled and editable after the Undo operation, matching its original and intended state

Reproducible with versions: 6000.3.12f1, 6000.4.1f1, 6000.5.0b1, 6000.6.0a1

Can’t test with versions: 6000.0.71f1 (Background foldout not available)

Tested on (OS): macOS 26.3 (M1)

Notes:

• The issue can be really inconsistant to reproduce and might require different combination of the selected values. But repeating selection and undo operation a few times should successfully reproduce the issue
• The property does not lock when normally interacting with the Scale Mode buttons; it only enters this broken locked state when triggered by the Undo system