Inkscape 0.92 in Ubuntu 18.04
I have detached a toolbox (the layers toolbox) and made it a free floating window, but then it is impossible to redock it in the right toolbox dock in Inkscape
Don’t drag with the window bar. Put your mouse on the panel title underneath, and drag from there.
Also, aim for the top of the right hand panel, or Inkscape may put your toolboxes in two columns.