fix(grid-overlay): write JSON defaults only on new file or Restore Defaults

- Add `is_new_file()` to config_file: tracks whether the config file
  was absent at load time; set true when load_from_file returns no data
- ux-window.cpp: gate the grid set_nested calls on is_new_file(), so
  defaults are written only when the JSON file is first created
- viewer.cpp: populate grid defaults in temp_cfg when Restore Defaults
  is pressed, so the section is (re)created on OK/Apply

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Evgeni Raikhel

common/rs-config.cpp

@@ -88,16 +88,20 @@ config_file& config_file::instance()
 config_file::config_file( std::string const & filename )
     : _filename( filename )
 {
+    bool file_loaded = false;
     try
     {
         auto j = rsutils::json_config::load_from_file( filename );
         if( j.exists() )
+        {
             _j = std::move( j );
+            file_loaded = true;
+        }
     }
     catch(...)
     {
-
     }
+    _is_new_file = !file_loaded;
 }
 
 void config_file::save()

common/rs-config.h

@@ -78,13 +78,15 @@ namespace rs2
         }
 
         bool contains(const char* key) const;
-        
+
         void save(const char* filename);
 
         void reset();
 
         void remove(const char* key);
 
+        bool is_new_file() const { return _is_new_file; }
+
         static config_file& instance();
 
         // Retrieves a value from a nested JSON structure using dot notation
@@ -190,5 +192,6 @@ namespace rs2
         std::map<std::string, std::string> _defaults;
         std::string _filename;
         rsutils::json _j;
+        bool _is_new_file = false;
     };
 }

common/ux-window.cpp

@@ -88,15 +88,16 @@ namespace rs2
         config_file::instance().set_default(configurations::viewer::hwlogger_xml, "./HWLoggerEvents.xml");
 
 #ifdef BUILD_VIEWPORT_GRID_OVERLAY
+        if( config_file::instance().is_new_file() )
         {
             namespace cfg = configurations::viewer::viewport_grid;
             auto& cf = config_file::instance();
-            cf.set_nested_default( cfg::horizontal_lines, 1   );
-            cf.set_nested_default( cfg::vertical_lines,   1   );
-            cf.set_nested_default( cfg::line_width,       1   );
-            cf.set_nested_default( cfg::line_color_r,     255 );
-            cf.set_nested_default( cfg::line_color_g,     255 );
-            cf.set_nested_default( cfg::line_color_b,     255 );
+            cf.set_nested( cfg::horizontal_lines, 1   );
+            cf.set_nested( cfg::vertical_lines,   1   );
+            cf.set_nested( cfg::line_width,       1   );
+            cf.set_nested( cfg::line_color_r,     255 );
+            cf.set_nested( cfg::line_color_g,     255 );
+            cf.set_nested( cfg::line_color_b,     255 );
         }
 #endif
 

common/viewer.cpp

@@ -2983,6 +2983,17 @@ namespace rs2
                     {
                         reload_required = true;
                         temp_cfg = config_file();
+#ifdef BUILD_VIEWPORT_GRID_OVERLAY
+                        {
+                            namespace cfg = configurations::viewer::viewport_grid;
+                            temp_cfg.set_nested( cfg::horizontal_lines, 1   );
+                            temp_cfg.set_nested( cfg::vertical_lines,   1   );
+                            temp_cfg.set_nested( cfg::line_width,       1   );
+                            temp_cfg.set_nested( cfg::line_color_r,     255 );
+                            temp_cfg.set_nested( cfg::line_color_g,     255 );
+                            temp_cfg.set_nested( cfg::line_color_b,     255 );
+                        }
+#endif
                     }
                     ImGui::SameLine();
                     if (ImGui::Button(" Export Settings "))