PCB Router: return to original active layer after routing

Fixes: https://gitlab.com/kicad/code/kicad/-/issues/12313

pcbnew/router/router_tool.cpp

@@ -1129,6 +1129,7 @@ bool ROUTER_TOOL::prepareInteractive()
         return false;
     }
 
+    m_originalActiveLayer = editFrame->GetActiveLayer();
     editFrame->SetActiveLayer( ToLAYER_ID( routingLayer ) );
 
     if( !getView()->IsLayerVisible( routingLayer ) )
@@ -1188,6 +1189,7 @@ bool ROUTER_TOOL::finishInteractive()
     m_startItem = nullptr;
     m_endItem   = nullptr;
 
+    frame()->SetActiveLayer( m_originalActiveLayer );
     UpdateMessagePanel();
     frame()->GetCanvas()->SetCurrentCursor( KICURSOR::ARROW );
     controls()->SetAutoPan( false );

pcbnew/router/router_tool.h

@@ -90,6 +90,7 @@ private:
     std::shared_ptr<ACTION_MENU> m_trackViaMenu;
 
     int                          m_lastTargetLayer;
+    PCB_LAYER_ID                 m_originalActiveLayer;
 };
 
 #endif