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, 41 insertions, 0 deletions

diff --git a/src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx b/src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx
index 4d47c3015..b14ac6a17 100644
--- a/src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx
+++ b/src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx
@@ -304,6 +304,47 @@ FL_EXPORT NSOpenGLContext *fl_mac_glcontext(GLContext rc) {
 }
 
 
+/* macOS offers only core contexts when using GL3. This forbids to add
+ FLTK widgets to a GL3-using Fl_Gl_Window because these widgets are drawn
+ with the GL1-based Fl_OpenGL_Graphics_Driver. The solution implemented here
+ is to create, with public function fl_mac_prepare_add_widgets_to_GL3_win(),
+ 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.
+ */
+
+class transparentGlWindow : public Fl_Gl_Window { // utility class
+  bool need_remove_opacity;
+public:
+  transparentGlWindow(int x, int y, int w, int h) : Fl_Gl_Window(x, y, w, h) {
+    mode(FL_RGB8 | FL_ALPHA | FL_SINGLE);
+    need_remove_opacity = true;
+  }
+  void show() {
+    Fl_Gl_Window::show();
+    if (need_remove_opacity) {
+      need_remove_opacity = false;
+      Fl_Cocoa_Window_Driver *d = Fl_Cocoa_Window_Driver::driver(this);
+      d->remove_gl_context_opacity((NSOpenGLContext*)context());
+    }
+  }
+};
+
+
+Fl_Gl_Window *fl_mac_prepare_add_widgets_to_GL3_win(Fl_Gl_Window *gl3win) {
+  gl3win->begin();
+  transparentGlWindow *transp = new transparentGlWindow(0, 0,
+                        gl3win->w(), gl3win->h());
+  gl3win->end();
+  if (!gl3win->resizable()) gl3win->resizable(gl3win);
+  if (gl3win->shown()) {
+    transp->show();
+    gl3win->make_current();
+  }
+  return transp;
+}
+
+
 class Fl_Gl_Cocoa_Plugin : public Fl_Cocoa_Plugin {
 public:
   Fl_Gl_Cocoa_Plugin() : Fl_Cocoa_Plugin(name()) { }