*Steps to reproduce:*
# Create and open an empty project
# Install com.unity.behavior
# Create and open a behavior graph (Assets > Create > Behavior > Behavior Graph)
# Create a GameObject blackboard variable
# Add a "Set Variable Value" node
# Assign the GameObject blackboard variable to the first field of the "Set Variable Value" node
# Create a prefab object by dragging a game object from the hierarchy to the project window
# 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|https://discussions.unity.com/t/15-minor-issues-of-behavior-graph/1590578] – they were using scriptable objects as an example
* Dragging onto the blackboard overlay *is* possible