Blackboard window Horizontal scrollbar does not move to the right side on clicking on Right Arrow.

Steps to repro:
1. Create a new HDRP template project.
2. Project Window > Click on "+" > Shader > PBR Graph.
3. Open Newly Created Shader Graph > Add Vector1 Property.
4. Blackboard window > Click on Right Directional Arrow in the Horizontal scrollbar.

Actual Result:
Horizontal scrollbar does not move to the right side.

Expected Result:
Horizontal scrollbar should move to the right side

Reproducible in:
2020.2.0a14, 2020.2.0a12, 2020.1.0a23 with Package Version 9.0.0, 8.1.0, 8.0.1.

Environment:
Occurring on Windows 10 & Mac 10.15.