Steps to reproduce:

1. Create and open an empty project
2. Install com.unity.behavior
3. Create and open a behavior graph (Assets > Create > Behavior > Behavior Graph)
4. Create a GameObject blackboard variable
5. Add a "Set Variable Value" node
6. Assign the GameObject blackboard variable to the first field of the "Set Variable Value" node
7. Create a prefab object by dragging a game object from the hierarchy to the project window
8. Drag the prefab onto the second field of the "Set Variable Value" node

Actual results: It is not possible to drag from the project window onto nodes in the graph window

Expected results: It is possible to drag from the project window onto nodes in the graph window

Reproducible with versions: 
1.0.7

Tested on (OS): MacOS

Notes:

• Brought up in Discussions forum here – they were using scriptable objects as an example
• Dragging onto the blackboard overlay is possible