This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
After dragging a widget around, a horizontal box will no longer accept children
Expected: The plus button would continue to add tabs
Result: After dragging the window around, the horizontal box will no longer accept children
Head over to the existing Questions & Answers thread and let us know what's up.