From ec150893533c3619bee4e389ac49994353a16b10 Mon Sep 17 00:00:00 2001
From: Efi Fogel <efifogel@gmail.com>
Date: Sun, 12 Mar 2023 14:59:40 +0200
Subject: [PATCH] Fixed handling unbounded curves that are both infinite on the
 left and infinite at the top

---
 .../Arr_unb_planar_construction_helper.h          | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_topology_traits/Arr_unb_planar_construction_helper.h b/Arrangement_on_surface_2/include/CGAL/Arr_topology_traits/Arr_unb_planar_construction_helper.h
index e0dc96dd823..ac24d67470b 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_topology_traits/Arr_unb_planar_construction_helper.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_topology_traits/Arr_unb_planar_construction_helper.h
@@ -272,6 +272,21 @@ before_handle_event(Event* event)
     if (m_prev_minus_inf_x_event != nullptr)
       m_prev_minus_inf_x_event->set_halfedge_handle(m_lh->next());
     m_prev_minus_inf_x_event = event;
+
+    // If the event lies also on the top boundary, associate all curve indices
+    // of subcurves that "see" m_th from below with the top fictitious halfedge
+    // (m_th->next()).
+    if (ps_y == ARR_TOP_BOUNDARY) {
+      if (m_he_ind_map_p != nullptr) {
+        Indices_list& list_ref = (*m_he_ind_map_p)[m_th];
+        list_ref.clear();
+        list_ref.splice(list_ref.end(), m_subcurves_at_ubf);
+      }
+      else {
+        m_subcurves_at_ubf.clear();
+      }
+      CGAL_assertion(m_subcurves_at_ubf.empty());
+    }
     return;
 
    case ARR_RIGHT_BOUNDARY: