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.
Any undocked windows that have their tab hidden cannot be unhidden and the window cannot be moved. This is problematic because one this is done even if you close the window and reopen it via the Window menu the tab is still hidden. The only way to reset this is to use Window > Reset Layout to go back to default configuration.
Video attached to demonstrate the issue.
*This was brought up after the ticket was submitted. In Editor's like the Animation Tools (formerly Persona), if you undock the viewport there for any of the windows you cannot reset this even with Window > Reset Layout.*
1. Go to Window > select any panel here like Statistics, levels, details, etc.
2. When the window opens, Right-click on the tab and select "Hide Tab"
3. Now try to unhide the tab by clicking the Yellow triangle.
Results: Once an undocked window has a hidden tab it cannot be unhidden and the window cannot be moved.
Workaround: Go to Window > Reset Layout and this will restore the original settings for all windows and panels to the default configuration.
Head over to the existingAnswerHub thread and let us know what's up.