Slider handle disappears on sliding to the extreme left for the Brush size

Steps to Repro:

1. Create a new project
2. Hierarchy > right click > 3D > Terrain
3. Select Terrain and go to Inspector
4. Select Brush
5. Select Raise or Lower Terrain
5. Drag Brush size slider to the extreme left

Actual Result:
- Slider handle disappears
- Values goes less than 1

Expected Result: Value set to 1

Reproducible on :
2019.3.0a1, 2019.2.0a14, 2019.1.0f2

Working fine on:
2018.3.0f2

Environment
Windows 10 and Mac 10.14