Prevent creation of invalid polyline / polygon

python/PyQt6/core/auto_generated/layout/qgslayoutitemnodeitem.sip.in

@@ -111,6 +111,14 @@ Deselects any selected nodes.
     virtual double estimatedFrameBleed() const;
 
 
+
+    virtual bool isValid() const;
+%Docstring
+Can be reimplemented in subclasses. Typically a polyline is valid if it has at least 2 distinct nodes, while
+a polygon is valid if it has at least 3 distinct nodes.
+
+.. versionadded:: 3.40
+%End
   protected:
 
     QgsLayoutNodesItem( QgsLayout *layout );

python/PyQt6/core/auto_generated/layout/qgslayoutitempolygon.sip.in

@@ -53,6 +53,8 @@ The caller takes responsibility for deleting the returned object.
 
     virtual QgsGeometry clipPath() const;
 
+    virtual bool isValid() const;
+
 
     QgsFillSymbol *symbol();
 %Docstring

python/PyQt6/core/auto_generated/layout/qgslayoutitempolyline.sip.in

@@ -55,6 +55,8 @@ The caller takes responsibility for deleting the returned object.
 
     virtual QPainterPath shape() const;
 
+    virtual bool isValid() const;
+
 
     QgsLineSymbol *symbol();
 %Docstring

python/core/auto_generated/layout/qgslayoutitemnodeitem.sip.in

@@ -111,6 +111,14 @@ Deselects any selected nodes.
     virtual double estimatedFrameBleed() const;
 
 
+
+    virtual bool isValid() const;
+%Docstring
+Can be reimplemented in subclasses. Typically a polyline is valid if it has at least 2 distinct nodes, while
+a polygon is valid if it has at least 3 distinct nodes.
+
+.. versionadded:: 3.40
+%End
   protected:
 
     QgsLayoutNodesItem( QgsLayout *layout );

python/core/auto_generated/layout/qgslayoutitempolygon.sip.in

@@ -53,6 +53,8 @@ The caller takes responsibility for deleting the returned object.
 
     virtual QgsGeometry clipPath() const;
 
+    virtual bool isValid() const;
+
 
     QgsFillSymbol *symbol();
 %Docstring

python/core/auto_generated/layout/qgslayoutitempolyline.sip.in

@@ -55,6 +55,8 @@ The caller takes responsibility for deleting the returned object.
 
     virtual QPainterPath shape() const;
 
+    virtual bool isValid() const;
+
 
     QgsLineSymbol *symbol();
 %Docstring

src/core/layout/qgslayoutitemnodeitem.h

@@ -113,6 +113,15 @@ class CORE_EXPORT QgsLayoutNodesItem: public QgsLayoutItem
     // rather than the item's pen
     double estimatedFrameBleed() const override;
 
+
+    /**
+     * Can be reimplemented in subclasses. Typically a polyline is valid if it has at least 2 distinct nodes, while
+     * a polygon is valid if it has at least 3 distinct nodes.
+     *
+     * \since QGIS 3.40
+     */
+    virtual bool isValid() const { return true; };
+
   protected:
 
     /**

src/core/layout/qgslayoutitempolygon.cpp

@@ -129,6 +129,23 @@ QgsGeometry QgsLayoutItemPolygon::clipPath() const
   return QgsGeometry::fromQPolygonF( path );
 }
 
+
+bool QgsLayoutItemPolygon::isValid() const
+{
+  QList<QPointF> uniquePoints;
+  int seen = 0;
+  for ( QPointF point : mPolygon )
+  {
+    if ( !uniquePoints.contains( point ) )
+    {
+      uniquePoints.append( point );
+      if ( ++seen > 2 )
+        return true;
+    }
+  }
+  return false;
+}
+
 QgsFillSymbol *QgsLayoutItemPolygon::symbol()
 {
   return mPolygonStyleSymbol.get();

src/core/layout/qgslayoutitempolygon.h

@@ -58,6 +58,7 @@ class CORE_EXPORT QgsLayoutItemPolygon: public QgsLayoutNodesItem
     bool accept( QgsStyleEntityVisitorInterface *visitor ) const override;
     QgsLayoutItem::Flags itemFlags() const override;
     QgsGeometry clipPath() const override;
+    bool isValid() const override;
 
     /**
      * Returns the fill symbol used to draw the shape.

src/core/layout/qgslayoutitempolyline.cpp

@@ -85,17 +85,17 @@ bool QgsLayoutItemPolyline::_removeNode( const int index )
 {
   if ( index < 0 || index >= mPolygon.size() )
     return false;
-
+  
   mPolygon.remove( index );
 
   if ( mPolygon.size() < 2 )
     mPolygon.clear();
   else
   {
-    int newSelectNode = index;
-    if ( index >= mPolygon.size() )
-      newSelectNode = mPolygon.size() - 1;
-    setSelectedNode( newSelectNode );
+  int newSelectNode = index;
+  if ( index >= mPolygon.size() )
+    newSelectNode = mPolygon.size() - 1;
+  setSelectedNode( newSelectNode );
   }
 
   return true;
@@ -344,6 +344,22 @@ QPainterPath QgsLayoutItemPolyline::shape() const
   return strokedOutline;
 }
 
+bool QgsLayoutItemPolyline::isValid() const
+{
+  QList<QPointF> uniquePoints;
+  int seen = 0;
+  for (QPointF point: mPolygon)
+  {
+    if( !uniquePoints.contains( point ) )
+    {
+      uniquePoints.append( point );
+      if (++seen > 1)
+        return true;
+    }
+  }
+  return false;
+}
+
 QgsLineSymbol *QgsLayoutItemPolyline::symbol()
 {
   return mPolylineStyleSymbol.get();

src/core/layout/qgslayoutitempolyline.h

@@ -66,6 +66,7 @@ class CORE_EXPORT QgsLayoutItemPolyline: public QgsLayoutNodesItem
     QIcon icon() const override;
     QString displayName() const override;
     QPainterPath shape() const override;
+    bool isValid() const override;
 
     /**
      * Returns the line symbol used to draw the shape.

src/gui/layout/qgslayoutviewtooladdnodeitem.cpp

@@ -74,7 +74,15 @@ void QgsLayoutViewToolAddNodeItem::layoutPressEvent( QgsLayoutViewMouseEvent *ev
       return;
 
     if ( QgsLayoutNodesItem *nodesItem = qobject_cast< QgsLayoutNodesItem * >( item ) )
+    {
       nodesItem->setNodes( mPolygon );
+      if ( !nodesItem->isValid() )
+      {
+        nodesItem->deleteLater();
+        mRubberBand.reset();
+        return;
+      }
+    }
 
     layout()->addLayoutItem( item );
     layout()->setSelectedItem( item );