Be more pedantic about RECURSE_MODE.

Also fixes an invalid iterator bug.

Also adds mirroring support for PCB_GROUPs.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21107

pcbnew/tools/edit_tool.cpp

@@ -2190,6 +2190,7 @@ const std::vector<KICAD_T> EDIT_TOOL::MirrorableItems = {
         PCB_TRACE_T,
         PCB_ARC_T,
         PCB_VIA_T,
+        PCB_GROUP_T,
         PCB_GENERATOR_T,
 };
 
@@ -2251,7 +2252,7 @@ int EDIT_TOOL::Mirror( const TOOL_EVENT& aEvent )
             continue;
 
         if( !item->IsNew() && !item->IsMoving() )
-            commit->Modify( item );
+            commit->Modify( item, nullptr, RECURSE_MODE::RECURSE );
 
         // modify each object as necessary
         switch( item->Type() )
@@ -2287,6 +2288,10 @@ int EDIT_TOOL::Mirror( const TOOL_EVENT& aEvent )
             static_cast<PCB_TRACK*>( item )->Mirror( mirrorPoint, flipDirection );
             break;
 
+        case PCB_GROUP_T:
+            static_cast<PCB_GROUP*>( item )->Mirror( mirrorPoint, flipDirection );
+            break;
+
         case PCB_GENERATOR_T:
             static_cast<PCB_GENERATOR*>( item )->Mirror( mirrorPoint, flipDirection );
             break;