commit da66e21e1d5826639da2b8d3bd5ccd27920a4aad
Author:     ManoloFLTK <41016272+ManoloFLTK@users.noreply.github.com>
AuthorDate: Thu Sep 29 09:39:21 2022 +0200
Commit:     ManoloFLTK <41016272+ManoloFLTK@users.noreply.github.com>
CommitDate: Thu Sep 29 09:39:21 2022 +0200

    Support of FLTK widgets in OpenGL 3 windows - cont'd.
    This commit allows to switch between FL_DOUBLE / FL_SINGLE modes
    in widget-containing GL3 windows.
    Demo program examples/OpenGL3test is modified to show FLTK widgets
    even if the platform does not support OpenGL 3.

 FL/gl.h                                         |  1 +
 examples/OpenGL3test.cxx                        | 22 ++++++++++++++--------
 src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx |  1 -
 src/gl_draw.cxx                                 | 14 +++++++++-----
 4 files changed, 24 insertions(+), 14 deletions(-)

diff --git FL/gl.h FL/gl.h
index 4579be2..a30d2ea 100644
--- FL/gl.h
+++ FL/gl.h
@@ -91,6 +91,7 @@ FL_EXPORT void gl_draw(const char*, int x, int y, int w, int h, Fl_Align);
 FL_EXPORT void gl_measure(const char*, int& x, int& y);
 FL_EXPORT void gl_texture_pile_height(int max);
 FL_EXPORT int  gl_texture_pile_height();
+FL_EXPORT void gl_texture_reset();
 
 FL_EXPORT void gl_draw_image(const uchar *, int x,int y,int w,int h, int d=3, int ld=0);
 
diff --git examples/OpenGL3test.cxx examples/OpenGL3test.cxx
index 40d0bf7..d9216c4 100644
--- examples/OpenGL3test.cxx
+++ examples/OpenGL3test.cxx
@@ -30,7 +30,7 @@
 #  endif
 #  include <GL/glew.h>
 #endif
-
+#include <FL/gl.h> // for gl_texture_reset()
 
 void add_output(const char *format, ...);
 
@@ -50,8 +50,7 @@ public:
     gl_version_major = 0;
   }
   void draw(void) {
-    if (gl_version_major < 3) return;
-    if (!shaderProgram) {
+    if (gl_version_major >= 3 && !shaderProgram) {
       GLuint  vs;
       GLuint  fs;
       int Mslv, mslv; // major and minor version numbers of the shading language
@@ -135,9 +134,11 @@ public:
     }
     glClearColor(0.08, 0.8, 0.8, 1.0);
     glClear(GL_COLOR_BUFFER_BIT);
-    GLfloat p[]={0,0};
-    glUniform2fv(positionUniform, 1, (const GLfloat *)&p);
-    glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
+    if (shaderProgram) {
+      GLfloat p[]={0,0};
+      glUniform2fv(positionUniform, 1, (const GLfloat *)&p);
+      glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
+    }
     Fl_Gl_Window::draw(); // Draw FLTK child widgets.
   }
   virtual int handle(int event) {
@@ -158,7 +159,12 @@ public:
       const uchar *glv = glGetString(GL_VERSION);
       add_output("GL_VERSION=%s\n", glv);
       sscanf((const char *)glv, "%d", &gl_version_major);
-      if (gl_version_major < 3) add_output("\nThis platform does not support OpenGL V3\n\n");
+      if (gl_version_major < 3) {
+        add_output("\nThis platform does not support OpenGL V3 :\n"
+                   "FLTK widgets will appear but the programmed "
+                   "rendering pipeline will not run.\n");
+        mode(mode() & !FL_OPENGL3);
+      }
       redraw();
     }
 
@@ -181,7 +187,7 @@ public:
     }
     return retval;
   }
-  void reset(void) { shaderProgram = 0; }
+  void reset(void) { shaderProgram = 0; gl_texture_reset(); }
 };
 
 
diff --git src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx
index bb4c53d..3ee281b 100644
--- src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx
+++ src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.cxx
@@ -27,7 +27,6 @@
 #include <FL/Fl_Image_Surface.H>
 #include <dlfcn.h>
 
-extern void gl_texture_reset();
 
 #ifdef __OBJC__
 @class NSOpenGLPixelFormat;
diff --git src/gl_draw.cxx src/gl_draw.cxx
index 3104f16..b786b83 100644
--- src/gl_draw.cxx
+++ src/gl_draw.cxx
@@ -319,11 +319,6 @@ int gl_texture_fifo::already_known(const char *str, int n)
 
 static gl_texture_fifo *gl_fifo = NULL; // points to the texture pile class instance
 
-void gl_texture_reset()
-{
-  if (gl_fifo) gl_texture_pile_height(gl_texture_pile_height());
-}
-
 
 // Cross-platform implementation of the texture mechanism for text rendering
 // using textures with the alpha channel only.
@@ -465,6 +460,15 @@ int gl_texture_pile_height(void)
   return gl_fifo->size();
 }
 
+/** To call after GL operations that may invalidate textures used to draw text in GL scenes
+ (e.g., switch between FL_DOUBLE / FL_SINGLE modes).
+ */
+void gl_texture_reset()
+{
+  if (gl_fifo) gl_texture_pile_height(gl_texture_pile_height());
+}
+
+
 /**
  Changes the maximum height of the pile of pre-computed string textures