diff --git a/src/fluency/ui/sketch_widget.py b/src/fluency/ui/sketch_widget.py
index dde4408..1147b61 100644
--- a/src/fluency/ui/sketch_widget.py
+++ b/src/fluency/ui/sketch_widget.py
@@ -1474,6 +1474,9 @@ class Sketch2DWidget(QWidget):
                     d1.is_construction = True
                     d2.is_construction = True
 
+        # Rebuild widget tracking lists so new entities (e.g. diagonal
+        # construction lines) are visible in paintEvent.
+        self._rebuild_from_sketch()
         self._solve_and_sync()
         sketch_updated = getattr(self, "sketch_updated", None)
         if sketch_updated is not None: