Previously there was a bug where it was tabified when a floater was focused.
In addition to correcting the placement of the first slave window, it is made
opaque when a floater has focus. The master window has this behavior already,
and why not the slave too?
The primary use-case for behavior (going opaque when a floater has focus) is
dialogs that show "live changes" in the main window, such as the "Colorize..."
dialog in the gimp.