Add property button in blackboard window gets misaligned on resizing.

Steps to repro:
1. Create a new project.
2. Windows > Package Manager > Visual Effects Graph > Install.
3. Packages > Visual Effect Graph > Editor > Templates > Open Empty Particle System.
4. Select Blackboard window > try to resize.

Actual Result:
Add button gets misaligned on resizing.

Expected Result:
Add button is aligned on resizing.

Reproducible in:
2019.3.0a12, 2019.3.0a10.

Workaround:
Close the graph window and open again.

NOTE:
1. Visual Effects introduced in 2019.3.0a10.
2. Could not test on Mac due to https://fogbugz.unity3d.com/f/cases/1174796/

Environment:
Only on Windows.