[layouts] Fix legends do not respect reference point when auto sizing

Fixes #18269

Author: nyalldawson

src/core/layout/qgslayoutitemlegend.cpp

@@ -182,9 +182,9 @@ void QgsLayoutItemLegend::adjustBoxSize()
   QgsDebugMsg( QString( "width = %1 height = %2" ).arg( size.width() ).arg( size.height() ) );
   if ( size.isValid() )
   {
-    QRectF targetRect = QRectF( pos().x(), pos().y(), size.width(), size.height() );
+    QgsLayoutSize newSize = mLayout->convertFromLayoutUnits( size, sizeWithUnits().units() );
     //set new rect, respecting position mode and data defined size/position
-    attemptSetSceneRect( targetRect );
+    attemptResize( newSize );
   }
 }
 

tests/src/python/test_qgslayoutlegend.py

@@ -26,7 +26,10 @@
                        QgsProject,
                        QgsLayoutObject,
                        QgsProperty,
-                       QgsLayoutMeasurement)
+                       QgsLayoutMeasurement,
+                       QgsLayoutItem,
+                       QgsLayoutPoint,
+                       QgsLayoutSize)
 from qgis.testing import (start_app,
                           unittest
                           )
@@ -84,6 +87,24 @@ def testInitialSizeSymbolMapUnits(self):
         result, message = checker.testLayout()
         self.assertTrue(result, message)
 
+        # resize with non-top-left reference point
+        legend.setResizeToContents(False)
+        legend.setReferencePoint(QgsLayoutItem.LowerRight)
+        legend.attemptMove(QgsLayoutPoint(120, 90))
+        legend.attemptResize(QgsLayoutSize(50, 60))
+
+        self.assertEqual(legend.positionWithUnits().x(), 120.0)
+        self.assertEqual(legend.positionWithUnits().y(), 90.0)
+        self.assertAlmostEqual(legend.pos().x(), 70, -1)
+        self.assertAlmostEqual(legend.pos().y(), 30, -1)
+
+        legend.setResizeToContents(True)
+        legend.updateLegend()
+        self.assertEqual(legend.positionWithUnits().x(), 120.0)
+        self.assertEqual(legend.positionWithUnits().y(), 90.0)
+        self.assertAlmostEqual(legend.pos().x(), 91, -1)
+        self.assertAlmostEqual(legend.pos().y(), 71, -1)
+
         QgsProject.instance().removeMapLayers([point_layer.id()])
 
     def testResizeWithMapContent(self):