Cannot paste with the middle click. From GDB:

(gdb) bt 3
#0  Fl::paste (receiver=..., clipboard=0, type=0x5555556c46b3 "text/plain") at /home/ram/dev/dillo/fltk/git/src/Fl.cxx:1756
#1  0x000055555562a591 in Fl_Text_Editor::handle (this=0x5555557428e0, event=1) at /home/ram/dev/dillo/fltk/git/src/Fl_Text_Editor.cxx:743
#2  0x00005555555f37b9 in send (o=0x5555557428e0, event=1) at /home/ram/dev/dillo/fltk/git/src/Fl_Group.cxx:99
(More stack frames follow...)

Seems to be caused by FLTK rejecting any clipboard value except 1:

void Fl_Wayland_Screen_Driver::paste(Fl_Widget &receiver, int clipboard, const char *type) {
  if (clipboard != 1) return;
  ...
}

FLTK issue: https://github.com/fltk/fltk/issues/1621