I mentioned this a few betas back. But I'm not sure where to search if the bug is already in the system.
The same behavior happens in 6.11 RC 1. I have Groups List as Dockable locked on right hand side of the application window. It's inside the window frame enough that it does not obscure a vertical scroll bar. When I right click a group and select "show all posts" it shows the posts fine, but it leaves this fly-out open until I click the mouse: