[vertex tool] Fix vertex addition to polygon's first segment (fixes #20774)

src/app/vertextool/qgsvertextool.cpp

@@ -172,18 +172,35 @@ class MatchCollectingFilter : public QgsPointLocator::MatchFilter
     {
       if ( match.distance() > 0 )
         return false;
-      matches.append( match );
 
       // there may be multiple points at the same location, but we get only one
       // result... the locator API needs a new method verticesInRect()
       QgsGeometry matchGeom = vertextool->cachedGeometry( match.layer(), match.featureId() );
+      bool isPolygon = matchGeom.type() == QgsWkbTypes::PolygonGeometry;
+      QgsVertexId polygonRingVid;
       QgsVertexId vid;
       QgsPoint pt;
       while ( matchGeom.constGet()->nextVertex( vid, pt ) )
       {
         int vindex = matchGeom.vertexNrFromVertexId( vid );
-        if ( pt.x() == match.point().x() && pt.y() == match.point().y() && vindex != match.vertexIndex() )
+        if ( pt.x() == match.point().x() && pt.y() == match.point().y() )
         {
+          if ( isPolygon )
+          {
+            // for polygons we need to handle the case where the first vertex is matching because the
+            // last point will have the same coordinates and we would have a duplicate match which
+            // would make subsequent code behave incorrectly (topology editing mode would add a single
+            // vertex twice)
+            if ( vid.vertex == 0 )
+            {
+              polygonRingVid = vid;
+            }
+            else if ( vid.ringEqual( polygonRingVid ) && vid.vertex == matchGeom.constGet()->vertexCount( vid.part, vid.ring ) - 1 )
+            {
+              continue;
+            }
+          }
+
           QgsPointLocator::Match extra_match( match.type(), match.layer(), match.featureId(),
                                               0, match.point(), vindex );
           matches.append( extra_match );

tests/src/app/testqgsvertextool.cpp

@@ -67,6 +67,7 @@ class TestQgsVertexTool : public QObject
     void testDeleteVertexTopo();
     void testAddVertexTopo();
     void testMoveEdgeTopo();
+    void testAddVertexTopoFirstSegment();
     void testActiveLayerPriority();
     void testSelectedFeaturesPriority();
 
@@ -642,6 +643,27 @@ void TestQgsVertexTool::testMoveEdgeTopo()
   QgsProject::instance()->setTopologicalEditing( false );
 }
 
+void TestQgsVertexTool::testAddVertexTopoFirstSegment()
+{
+  // check that when adding a vertex to the first segment of a polygon's ring with topo editing
+  // enabled, the geometry does not get corrupted (#20774)
+
+  QgsProject::instance()->setTopologicalEditing( true );
+
+  mouseClick( 5.5, 1, Qt::LeftButton );
+  mouseClick( 5, 2, Qt::LeftButton );
+
+  QCOMPARE( mLayerPolygon->undoStack()->index(), 2 );
+
+  QCOMPARE( mLayerPolygon->getFeature( mFidPolygonF1 ).geometry(), QgsGeometry::fromWkt( "POLYGON((4 1, 5 2, 7 1, 7 4, 4 4, 4 1))" ) );
+
+  mLayerPolygon->undoStack()->undo();
+
+  QCOMPARE( mLayerPolygon->getFeature( mFidPolygonF1 ).geometry(), QgsGeometry::fromWkt( "POLYGON((4 1, 7 1, 7 4, 4 4, 4 1))" ) );
+
+  QgsProject::instance()->setTopologicalEditing( false );
+}
+
 void TestQgsVertexTool::testActiveLayerPriority()
 {
   // check that features from current layer get priority when picking points