This has been observed with Windows, in two hosts, and on two separate machines: Reaper and FL Studio 12. On a fresh install of redux, when pressing the 'editor' button to expand the editor, redux enlarges the window. The default window size is large enough that the resize widget in the lower right of the redux window is inaccessible, and redux refuses to be resized any other way. (On one of the machines this made the lower part of the redux window inaccessible in editor mode.)
In one case I was able to detach the redux window in FL Studio and get at the widget, on another machine I had to use the Windows10 display scaling set to 175% in Reaper to force scroll bars to appear, and then use redux's lower-right resize widget, and then revert the display scaling.
Once the redux window has been made smaller, it remembers its size, so this fiddling only needs to be done once, but ideally it would be best not needed at all. A fix should be easy, I imagine.