Fast Light Toolkit - user interface designer This package provides Fluid, an interactive GUI designer for FLTK, allowing graphical development of FLTK-based user interfaces. It natively works with (textual) .fl files, but also includes rough support for reading .fd files produced by fdesign (in libforms-bin). The Fast Light Toolkit (FLTK) is a cross-platform graphical user interface toolkit originally based on libForms.