Fix of #5537 - GUI_App::post_init could be in some cases called before GUI_App::init_openg when GCode viewer is used.

The solution adds a status flag to GUI_App::init_openg and ensures that GUI_App::post_init isn't called before initialization in GUI_App::init_openg is done.

src/slic3r/GUI/GUI_App.cpp

@@ -669,7 +669,13 @@ wxGLContext* GUI_App::init_glcontext(wxGLCanvas& canvas)
 
 bool GUI_App::init_opengl()
 {
+#ifdef __linux__
+    bool status = m_opengl_mgr.init_gl();
+    m_opengl_initialized = true;
+    return status;
+#else
     return m_opengl_mgr.init_gl();
+#endif
 }
 
 void GUI_App::init_app_config()
@@ -916,7 +922,14 @@ bool GUI_App::on_init_inner()
         this->obj_manipul()->update_if_dirty();
 
         static bool update_gui_after_init = true;
+
+        // An ugly solution to GH #5537 in which GUI_App::init_opengl (normally called from events wxEVT_PAINT
+        // and wxEVT_SET_FOCUS before GUI_App::post_init is called) wasn't called before GUI_App::post_init and OpenGL wasn't initialized.
+#ifdef __linux__
+        if (update_gui_after_init && m_opengl_initialized) {
+#else
         if (update_gui_after_init) {
+#endif
             update_gui_after_init = false;
 #ifdef WIN32
             this->mainframe->register_win32_callbacks();

src/slic3r/GUI/GUI_App.hpp

@@ -109,6 +109,9 @@ private:
     bool            m_app_conf_exists{ false };
     EAppMode        m_app_mode{ EAppMode::Editor };
     bool            m_is_recreating_gui{ false };
+#ifdef __linux__
+    bool            m_opengl_initialized{ false };
+#endif
 
     wxColour        m_color_label_modified;
     wxColour        m_color_label_sys;