Add button is clipped from below on the Add Tree, Texture, Grass Texture, Detail Mesh window, similarly, Import & Export button is clipped on Import and Export heightmap window on terrain

Steps to Repor:
1. Open attached or create a new project
2. Hierarchy window > right click
3. 3D object > terrain
4. Select paint trees tab > Edit tree
5. Add Tree

Actual Result:
1. Observe the placement of the add button
2. The button is getting clipped from below

Expected Result:
1. There should be enough margin space from the right-hand side and below the button

Environment:
Windows and Mac

Occurring with:
2018.3.0a4, 2018.3.0a3, 2018.3.0a1

Working with:
2018.2.0b10