As already mentionned, the solution for this problem is to use the

Fl_Window::shape() member function. See :

    https://www.fltk.org/doc-1.3/classFl__Window.html#aeb3ffb4925bd90950de419f962bcf7f6

This feature is available in FLTK 1.4 and also starting from FLTK 1.3.3 provided the library is built with

    FL_ABI_VERSION = 10303  (or more)

Giving a window the shape of an Fl_ROUND_BOX is done in FLTK 1.4 to transiently

display the new value of the scale factor (type ctrl-+ (or cmd-+ on macOS)).

The code is found in function Fl_Screen_Driver::transient_scale_display() of

file Fl_Screen_Driver.cxx :

- lines 363-373 create an Fl_RGB_Image object containing the adequate shape information

- line 388 applies this shape to the window (and also set it without titlebar)

That should be enough.