How to reproduce:
1. Create a new project
2. Import the Shader Graph package from the Package Manager
3. Resize the Editor to make it similar to a standard laptop size (14-16 inch) or smaller
4. Set the Layout to 'Default' (Window -> Layouts -> Default)
5. In the Project window create a new Shader Graph (for example, Unlit Graph)
6. Double-click on the newly created Shader Graph to open it in the Shader Editor

Expected result: the Main Preview window doesn't cover other areas (Master Node, Blackboard) and its title bar is always visible
Actual result: the 'Main Preview' title bar and other areas are hidden behind the Main Preview window

Reproducible with: 2019.4.5f1 (7.3.1, 7.4.1), 2020.1.0f1 (8.2.0), 2020.2.0a19 (9.0.0-preview.34)
Could not test with: 2018.4.25f1 (4.10.0-preview) (Shader Editor is empty)