How to reproduce:
1. Create a new project
2. Create 'Terrain' GameObject
3. Select 'Terrain' GameObject in the Hierarchy
4. Click Paint Terrain button in the Inspector
5. Select Stoke's Brush Strength dropdown

Expected result: Brush Strength dropdown is expanded
Actual result: Nothing happens

Reproducible with:.2019.3.0a7 (1.1.2, 2.0.0, 2.0.2), 2019.3.0b11 (1.1.2, 2.0.0, 2.0.2), 2020.1.0a13 (1.1.2, 2.0.0, 2.0.2)
Not reproducible with: 2019.2.12f1, 2019.3.0a6
Could not test: 2017.4.34f1, 2018.4.13f1 Stroke Settings are not introduced

Note: Reproducible with Brush Strength, Brush Size, Brush Rotation dropdowns