Add layer button becomes unusable when "Add Terrain Layer" window shrinks horizontally

Steps to Repro:
1. Open attached or create a new project
2. Hierarchy > right click
3. 3D object > terrain
4. Select paint texture from paint tab
5. Edit terrain layer > add a layer
6. Shrink layer window horizontally

Actual Result:
1. Add a layer and close button disappears

Expected Result:
1. Buttons shouldn't disappear when the window is shrinked

Occurring with:
2018.3.0a6 (13fac50c9eea) , 2018.3.0a5, 2018.3.0a4

Note:
New Terrain tools are introduced from alpha 2018.3.0a4

Environment:
Windows and Mac