"Apply all" and "Revert All" button is clipped under override window

Steps to repro:
1. Create a new project
2. Darg "Main Camera" game object in "Project" window to create a prefab
3. Edit any property under the camera component
4. Open override drop down
5. Observe "Apply all" and "Revert All" button

Actual Result:
1. The buttons are clipped from below

Expected Result:
1. The button's shape should be consistent for the editor UI

Occurring with:
2018.3.0a9 (1671df2d9715) , 2018.3.0a8, 2018.3.0a6

Note:
Improved prefabs were introduced in 2018.3.0a6

Environment:
Windows and Mac