How to reproduce:
1. Open the attached Unity project.
2. Open "New Shader Graph".
3. Resize shader graph's editor window.

Expected result: Blackboard is still visible in the shader graph editor
Actual result: Blackboard disappears from the shader graph editor

Reproducible with -2019.1.0b3, 2018.1.0f1, 2018.1.0f2, 2018.2.0b3
Not reproducible with - 2018.1.0b10, 2018.1.0b13 (does not have "Render-pipelines.lightweight" package version "1.1.5-preview")

Note:
- Shader graph editor first appeared in 2018.1.0b10, so I could not test this issue on earlier Unity versions.
- Workaround: delete "Library" folder. The Blackboard will appear unless you resize the Shader Graph editor window again.
- Only reproduces with "Render-pipelines.lightweight" package version "1.1.5-preview".