Hierarchy v2: Unable to drag and drop to empty space of hierarchy window when open prefab in context or isolation

Steps to reproduce:

1. Select a prefab in hierarchy and right click and select open in context or isolation
2. drag and drop another prefab from project browser to the empty space of hierarchy window as a child of prefab root

Actual results: Drag and drop is not allowed 

Expected results: user should be able to drag and drop new prefab to empty space as a child 

Reproducible with versions: 6000.5/4/3

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): mac

Notes:

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