Add cross-platform support for adding widgets to an OpenGL3-based Fl_Gl_Window.

Under non-macOS platforms, the key is to call glUseProgram(0); after having used OpenGL 3
which allows to then use OpenGL 1 and draw FLTK widgets over the OpenGL3 scene.

Under macOS, this is impossible because macOS GL3 contexts are not compatible
with GL1. The solution implemented here is to create an additional Fl_Gl_Window
placed above and sized as the GL3-based window, to give it a non opaque,
GL1-based context, and to put the FLTK widgets in that additional window.

1 files changed, 24 insertions, 0 deletions

diff --git a/src/glut_compatibility.cxx b/src/glut_compatibility.cxx
index 0a5a6b30e..011f9290b 100644
--- a/src/glut_compatibility.cxx
+++ b/src/glut_compatibility.cxx
@@ -30,6 +30,9 @@
 #  include "Fl_Screen_Driver.H"
 #  include <FL/glut.H>
 #  define MAXWINDOWS 32
+#  ifndef GL_CURRENT_PROGRAM
+#    define GL_CURRENT_PROGRAM 0x8B8D  // from glew.h
+#  endif
 
 static Fl_Glut_Window *windows[MAXWINDOWS+1];
 
@@ -54,6 +57,27 @@ void Fl_Glut_Window::draw() {
   indraw = 1;
   if (!valid()) {reshape(pixel_w(),pixel_h()); valid(1);}
   display();
+  if (children()) {
+    if ((mode() & FL_OPENGL3)) {
+#ifndef __APPLE__
+      typedef void (*glUseProgram_type)(GLint);
+      static glUseProgram_type glUseProgram_f = NULL;
+      if (!glUseProgram_f) {
+        Fl_Gl_Window_Driver *dr = Fl_Gl_Window_Driver::driver(this);
+        glUseProgram_f = (glUseProgram_type)dr->GetProcAddress("glUseProgram");
+      }
+      GLint current_prog = 0;
+      glGetIntegerv(GL_CURRENT_PROGRAM, &current_prog);
+      // Switch from GL3-style to GL1-style drawing;
+      // good under Windows, X11 and Wayland; impossible under macOS.
+      glUseProgram_f(0);
+      // Draw FLTK child widgets
+      Fl_Gl_Window::draw();
+      // Switch back to GL3-style drawing
+      glUseProgram_f((GLuint)current_prog);
+#endif // ! __APPLE__
+    } else Fl_Gl_Window::draw(); // Draw FLTK child widgets
+  }
   indraw = 0;
 }