Drag and drop item between content areas is inconsistent

Description

Steps to reproduce

1. Create a product page.
2. Drag and drop a block B1 from the assets pane to the large content area.
3. Drag and drop B1 from the large content area to a small content area.
4. Repeat step 3 two or three times.

Expected: B1 is moved from the large content area to the small content area, as in CMS 7.5 and CMS 8.

Actual: Drag and drop the first time > item copied from one content area to the other.
Drag and drop the second time > item copied from one content area to the other.
Drag and drop the third time > item _moved _from one content area to the other.