Cannot not add child to horizontal box after drag and drop

After dragging a widget around, a horizontal box will no longer accept children

1. Open the attached project
2. Open "NewMap"
3. Play in Editor
4. Click the add window button
5. Click the Plus button to add a tab ( looks like [+])
6. Click the blue area to click and drag the widget
7. Let go of the widget
8. Click the plus button again ( looks like [+])
9. Notice that no more tabs can be added after drag and drop

Expected: The plus button would continue to add tabs
Result: After dragging the window around, the horizontal box will no longer accept children