How to reproduce it:
1. Open the user's attached project 'UI Window Bug.zip'
2. In the Main Menu bar click 'Window' > 'Example'
3. In the 'Example' window select and drag the split view's dragline and notice that labels' minimum height is maintained
4. Shrink the height of the 'Example' window
5. Observe the 'Example' window's labels

Expected result: Labels' minimum height is preserved, and both labels are visible in the window
Actual result: The label's minimum height is not preserved, and the bottom label is hidden

Reproducible with: 2020.2.0a16, 2020.2.2f1 (1.0.0-preview.14), 2020.2.4f1, 2021.1.0b6, 2021.2.0a4
Could not test with: 2018.4.31f1 (UIElements are not supported), 2019.4.20f1, 2020.2.0a15 (error concerning TwoPaneSplitView)