Fix invalid values for @map_extent_width/height (fix #15672)

(cherry-picked from c1cf896)

Author: nyalldawson

src/core/qgsexpressioncontext.cpp

@@ -735,8 +735,8 @@ QgsExpressionContextScope* QgsExpressionContextUtils::mapSettingsScope( const Qg
   scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_id", "canvas", true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_rotation", mapSettings.rotation(), true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_scale", mapSettings.scale(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_extent_width", mapSettings.extent().width(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_extent_height", mapSettings.extent().height(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_extent_width", mapSettings.visibleExtent().width(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_extent_height", mapSettings.visibleExtent().height(), true ) );
   QgsGeometry* centerPoint = QgsGeometry::fromPoint( mapSettings.visibleExtent().center() );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( "map_extent_center", QVariant::fromValue( *centerPoint ), true ) );
   delete centerPoint;