fix(LayoutLegend): Correctly fix position while painting

While painting, it happens that other code can modify current item
position, but this new position will be really used on the new draw,
not the current one. So we have to backup it early enough (at the very
beginning of paint) so we can translate to this new position at the
end of the paint method.

Fixes #37566 (Again)

src/core/layout/qgslayoutitemlegend.cpp

@@ -106,6 +106,8 @@ void QgsLayoutItemLegend::paint( QPainter *painter, const QStyleOptionGraphicsIt
   if ( !painter )
     return;
 
+  const QPointF oldPos = pos();
+
   ensureModelIsInitialized();
 
   if ( mFilterAskedForUpdate )
@@ -157,8 +159,6 @@ void QgsLayoutItemLegend::paint( QPainter *painter, const QStyleOptionGraphicsIt
   QgsLegendRenderer legendRenderer = createRenderer();
   legendRenderer.setLegendSize( mForceResize && mSizeToContents ? QSize() : rect().size() );
 
-  const QPointF oldPos = pos();
-
   //adjust box if width or height is too small
   if ( mSizeToContents )
   {

tests/src/python/test_qgslayoutlegend.py

@@ -1098,6 +1098,19 @@ def testReferencePoint(self):
             )
         )
 
+        # re-render with filtering to trigger mapHitTest which ends up by calling adjustBoxSize().
+        # These last change the box size and therefore position (because our reference point is lower left)
+        # So check that the legend position is according to what we expect (change also marker size
+        # so we are sure that it's size and position have correcly been updated)
+        legend.setLegendFilterByMapEnabled(True)
+        marker_symbol.setSize(10)
+
+        self.assertTrue(
+            self.render_layout_check(
+                'composer_legend_reference_point_newsize', layout
+            )
+        )
+
         QgsProject.instance().clear()
 
     def test_rulebased_child_filter(self):