On open I see a Leo window with several internal panels like I'm used to (Outline, Body, Tabs, Render) and 4 new brightly coloured panels (Test1, Test2,...).
The 4 test panels can be snapped off the main Leo window using min/max icon at top right by [x] and float independently anywhere on my desktop.
The floats can also be stacked all into the same panel, but only as a component of the main Leo window. They can't be stacked when in Float mode. Neither can the whole stack be floated. When floating r-click to show other panels doesn't work, that can only be done in main Leo window.
Oh thats interesting: [Body] and [Tabs] can be floated the same as [TestN] but [Render] cannot (maybe this isn't new). The main Leo window is always underneath the floats (not necessarily a bad thing).
I am not able to add any of the standard panels into a test panel, e.g. I can't move [Render] to [Test2]. Any new .leo files I open go to the main Leo window and not a Test.
Post edited to add:
Leo 6.1-devel, gui branch, build 6370998652
2019-08-26 08:24:34 -0500
Python 3.7.1, PyQt version 5.13.0
Windows 10 AMD64 (build 10.0.17134) SP0
-matt