[Layers] Drop zone is not limited correctly.

[mmuller]

Found in version gda54b36

Preconditions:
There is a Layer.

Reproduce steps:
Navigate to the Layers section.
Drag a Layer out of the drop zone.

Obtained:
The Layer can be dragged out of the drop zone.
The Layer cannot be dropped out of the drop zone.

Expected:
The Layer should not be dragged out of the drop zone.

http://recordit.co/abZfL3ztNS