From 9560740b8e1548fb328b29739f79e2f3472f9b29 Mon Sep 17 00:00:00 2001
From: ManoloFLTK <41016272+ManoloFLTK@users.noreply.github.com>
Date: Fri, 8 Nov 2024 13:00:51 +0100
Subject: [PATCH] Fix X11: test/gl_overlay stale overlay rendering (#1111)

---
 src/Fl_Gl_Window.cxx | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src/Fl_Gl_Window.cxx b/src/Fl_Gl_Window.cxx
index b645b8997..e81fed7fe 100644
--- a/src/Fl_Gl_Window.cxx
+++ b/src/Fl_Gl_Window.cxx
@@ -236,6 +236,33 @@ void Fl_Gl_Window::ortho() {
 */
 void Fl_Gl_Window::swap_buffers() {
 #if defined(USE_X11)
+  if (overlay) {
+    int wo = w(), ho = h();
+    GLint matrixmode;
+    GLfloat pos[4];
+    glGetIntegerv(GL_MATRIX_MODE, &matrixmode);
+    glGetFloatv(GL_CURRENT_RASTER_POSITION, pos);       // save original glRasterPos
+    glMatrixMode(GL_PROJECTION);                        // save proj/model matrices
+    glPushMatrix();
+    glLoadIdentity();
+    glMatrixMode(GL_MODELVIEW);
+    glPushMatrix();
+    glLoadIdentity();
+    glScalef(2.0f/wo, 2.0f/ho, 1.0f);
+    glTranslatef(-wo/2.0f, -ho/2.0f, 0.0f);         // set transform so 0,0 is bottom/left of Gl_Window
+    glRasterPos2i(0,0);                             // set glRasterPos to bottom left corner
+    {
+      // Emulate overlay by doing copypixels
+      glReadBuffer(GL_BACK);
+      glDrawBuffer(GL_FRONT);
+      glCopyPixels(0, 0, wo, ho, GL_COLOR);         // copy GL_BACK to GL_FRONT
+    }
+    glPopMatrix(); // GL_MODELVIEW                  // restore model/proj matrices
+    glMatrixMode(GL_PROJECTION);
+    glPopMatrix();
+    glMatrixMode(matrixmode);
+    glRasterPos3f(pos[0], pos[1], pos[2]);              // restore original glRasterPos
+  } else
   glXSwapBuffers(fl_display, fl_xid(this));
 #elif defined(WIN32)
 #  if HAVE_GL_OVERLAY