fix(layoutlegendwidget): correctly move legend nodes when they are filtered

Fixes #60383.

In a layout, when moving a legend node on a filtered legend, the
computed offset was wrong if some nodes were hidden between the node to
move and its destination.

The offset is now computed with the next/previous visible index.

Author: Djedouas

src/gui/layout/qgslayoutlegendwidget.cpp

@@ -726,9 +726,10 @@ void QgsLayoutLegendWidget::mColumnSpaceSpinBox_valueChanged( double d )
   }
 }
 
-static void _moveLegendNode( QgsLayerTreeLayer *nodeLayer, int legendNodeIndex, int offset )
+static void _moveLegendNode( QgsLayerTreeLayer *nodeLayer, int legendNodeIndex, int destLegendNodeIndex )
 {
   QList<int> order = QgsMapLayerLegendUtils::legendNodeOrder( nodeLayer );
+  const int offset = destLegendNodeIndex - legendNodeIndex;
 
   if ( legendNodeIndex < 0 || legendNodeIndex >= order.count() )
     return;
@@ -770,8 +771,17 @@ void QgsLayoutLegendWidget::mMoveDownToolButton_clicked()
   }
   else // legend node
   {
-    _moveLegendNode( legendNode->layerNode(), _unfilteredLegendNodeIndex( legendNode ), 1 );
-    mItemTreeView->layerTreeModel()->refreshLayerLegend( legendNode->layerNode() );
+    // get the next index, the one that will have to be above our index,
+    const QModelIndex nextIndex = index.siblingAtRow( index.row() + 1 );
+    if ( nextIndex.isValid() )
+    {
+      QgsLayerTreeModelLegendNode *nextLegendNode = mItemTreeView->index2legendNode( nextIndex );
+      if ( nextLegendNode )
+      {
+        _moveLegendNode( legendNode->layerNode(), _unfilteredLegendNodeIndex( legendNode ), _unfilteredLegendNodeIndex( nextLegendNode ) );
+        mItemTreeView->layerTreeModel()->refreshLayerLegend( legendNode->layerNode() );
+      }
+    }
   }
 
   mItemTreeView->setCurrentIndex( mItemTreeView->proxyModel()->mapFromSource( mItemTreeView->layerTreeModel()->index( sourceIndex.row() + 1, 0, parentIndex ) ) );
@@ -808,8 +818,17 @@ void QgsLayoutLegendWidget::mMoveUpToolButton_clicked()
   }
   else // legend node
   {
-    _moveLegendNode( legendNode->layerNode(), _unfilteredLegendNodeIndex( legendNode ), -1 );
-    mItemTreeView->layerTreeModel()->refreshLayerLegend( legendNode->layerNode() );
+    // get the previous index, the one that will have to be below our index,
+    const QModelIndex prevIndex = index.siblingAtRow( index.row() - 1 );
+    if ( prevIndex.isValid() )
+    {
+      QgsLayerTreeModelLegendNode *prevLegendNode = mItemTreeView->index2legendNode( prevIndex );
+      if ( prevLegendNode )
+      {
+        _moveLegendNode( legendNode->layerNode(), _unfilteredLegendNodeIndex( legendNode ), _unfilteredLegendNodeIndex( prevLegendNode ) );
+        mItemTreeView->layerTreeModel()->refreshLayerLegend( legendNode->layerNode() );
+      }
+    }
   }
 
   mItemTreeView->setCurrentIndex( mItemTreeView->proxyModel()->mapFromSource( mItemTreeView->layerTreeModel()->index( sourceIndex.row() - 1, 0, parentIndex ) ) );