Pcbnew: don't assume zones are in the UiOrder list

This can be false, for example when zones are being rescued,
even if the LSET is non-zero-sized.

(cherry picked from commit 1662d7f546)

Fixes: https://gitlab.com/kicad/code/kicad/-/issues/18553

pcbnew/zone.cpp

@@ -237,18 +237,32 @@ VECTOR2I ZONE::GetPosition() const
 PCB_LAYER_ID ZONE::GetLayer() const
 {
     if( m_layerSet.count() == 1 )
-        return m_layerSet.UIOrder()[0];
-    else
-        return UNDEFINED_LAYER;
+    {
+        return GetFirstLayer();
+    }
+    return UNDEFINED_LAYER;
 }
 
 
 PCB_LAYER_ID ZONE::GetFirstLayer() const
 {
-    if( m_layerSet.count() )
-        return m_layerSet.UIOrder()[0];
-    else
+    if( m_layerSet.count() == 0 )
+    {
         return UNDEFINED_LAYER;
+    }
+
+    const LSEQ uiLayers = m_layerSet.UIOrder();
+
+    // This can't use m_layerSet.count() because it's possible to have a zone on
+    // a rescue layer that is not in the UI order.
+    if( uiLayers.size() )
+    {
+        return uiLayers[0];
+    }
+
+    // If it's not in the UI set at all, just return the first layer in the set.
+    // (we know the count > 0)
+    return m_layerSet.Seq()[0];
 }
 
 

qa/tests/pcbnew/CMakeLists.txt

@@ -47,6 +47,7 @@ set( QA_PCBNEW_SRCS
     test_save_load.cpp
     test_tracks_cleaner.cpp
     test_triangulation.cpp
+    test_zone.cpp
     test_zone_filler.cpp
 
     drc/test_custom_rule_severities.cpp

qa/tests/pcbnew/test_zone.cpp

@@ -0,0 +1,83 @@
+/*
+ * This program source code file is part of KiCad, a free EDA CAD application.
+ *
+ * Copyright (C) 2024 KiCad Developers, see AUTHORS.txt for contributors.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, you may find one here:
+ * http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+ * or you may search the http://www.gnu.org website for the version 2 license,
+ * or you may write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
+ */
+
+#include <qa_utils/wx_utils/unit_test_utils.h>
+#include <pcbnew_utils/board_test_utils.h>
+
+#include <board.h>
+#include <zone.h>
+
+
+struct ZONE_TEST_FIXTURE
+{
+    BOARD m_board;
+};
+
+
+BOOST_FIXTURE_TEST_SUITE( Zone, ZONE_TEST_FIXTURE )
+
+BOOST_AUTO_TEST_CASE( SingleLayer )
+{
+    ZONE zone( &m_board );
+
+    zone.SetLayer( F_Cu );
+
+    BOOST_CHECK( zone.GetLayer() == F_Cu );
+    BOOST_CHECK( zone.GetLayer() == zone.GetFirstLayer() );
+
+    BOOST_CHECK( zone.IsOnCopperLayer() == true );
+}
+
+BOOST_AUTO_TEST_CASE( MultipleLayers )
+{
+    ZONE zone( &m_board );
+
+    zone.SetLayerSet( { F_Cu, B_Cu } );
+
+    // There is no "the" layer in a multi-layer zone
+    BOOST_CHECK( zone.GetLayer() == UNDEFINED_LAYER );
+    // ... but there is a first layer
+    BOOST_CHECK( zone.GetFirstLayer() == F_Cu );
+
+    BOOST_CHECK( zone.IsOnCopperLayer() == true );
+}
+
+/**
+ * During zone loading, the layer is set to Rescue if the layer is not found.
+ * This is not a UI-visible layer, so make sure it can still be retreived.
+ *
+ * https://gitlab.com/kicad/code/kicad/-/issues/18553
+ */
+BOOST_AUTO_TEST_CASE( RescuedLayers )
+{
+    ZONE zone( &m_board );
+
+    zone.SetLayer( Rescue );
+
+    BOOST_CHECK( zone.GetLayer() == Rescue );
+    BOOST_CHECK( zone.GetLayer() == zone.GetFirstLayer() );
+
+    BOOST_CHECK( zone.IsOnCopperLayer() == false );
+}
+
+BOOST_AUTO_TEST_SUITE_END()