Continuing investigation …

The commit mentionned above introduces a call to Fl::add_fd() in the constructor of class ExternalCodeEditor.

At least under macOS, this call implies to open the display. The blocking occurs while attempting to open the display.

One might consider that Fl::add_fd() should be avoided by fluid in command-line mode because the display

should not be opened in that mode. The problem is that there's a global object created at static initialization time :

    Fl_Code_Type Fl_Code_type; (see line 547 of fluid/Fl_Function_Type.cxx)

and type Fl_Code_Type contains a member variable of type ExternalCodeEditor.
Therefore, the constructor of class ExternalCodeEditor runs (and calls Fl::add_fd() since the recent commit)

at static initialization time, also when fluid is in command-line mode.