Create node window flickers on resizing.

Steps to repro:
1. Create a new HDRP template project
2. Project window > "+" > Shader > HDRP > StackLit Graph.
3. Open Graph > Right-click in the Graph Editor.
4. Resize Create node window.

Actual Result:
Create node window flickers on resizing.

Expected Result:
Create node window should not flicker on resizing.

Reproducible in:
2020.2.0a18, 2020.1.0b16, 2020.1.0b6 with 9.0.0-preview.33, 8.2.0, 8.1.0.

Environment:
Occurring on Windows 10 & Mac 10.15.