How to reproduce:
1. Open attached project "Blank Shadergraph Panel.zip"
2. In the Editor, make sure that Edit -> Project Settings -> Player -> Other Settings -> Graphics API is set to OpenGLCore
3. In the Hierarchy window, select "Separate Normal Standard" shader
4. In Shader Graph window, observe the Blackboard and Preview panels

Expected result: the contents of Blackboard and Preview panels are rendered/visible
Actual result: the contents of Blackboard and Preview panels are visually invisible

Reproducible with: 2019.1.9f1 (5.2.3, 5.7.2), 2019.3.0a1 (6.7.1), 2019.3.0a2 (6.7.1), 2019.3.0a8 (6.7.1)
Not reproducible with: 2018.4.3f1 (4.10.0) could not fully test due to only "Separate Normal Channel" shader opening up (other shaders are opened by the Visual Studio with some errors instead of the Shader Graph window)
Could not test with: 2017.4.29f1 LWRP and Shader Graph was not yet implemented

Workarounds:
1. Do not use OpenGL Graphics API with Shader Graph
2. Update the Shader Graph window by docking/re-docking it to the Editor (may crash, depending on the Unity version)