Move splitSelfTouchingOutlines to cacheTriangulation

The splitSelfTouchingOutlines call was unconditionally called in
Simplify but is only needed during triangulation when dealing with
self-touching or self-intersecting polygons. Move the call to
cacheTriangulation where it's actually used and remove it from
Simplify.

Simplify the test to directly check triangulated area against the
expected value instead of relying on the simplified polygon as a
reference.

libs/kimath/src/geometry/shape_poly_set.cpp

@@ -2175,7 +2175,6 @@ void SHAPE_POLY_SET::splitSelfTouchingOutlines()
 
 void SHAPE_POLY_SET::Simplify()
 {
-    splitSelfTouchingOutlines();
     splitCollinearOutlines();
 
     SHAPE_POLY_SET empty;
@@ -3234,7 +3233,10 @@ void SHAPE_POLY_SET::cacheTriangulation( bool aPartition, bool aSimplify,
             flattened.ClearArcs();
 
             if( flattened.HasHoles() || flattened.IsSelfIntersecting() )
+            {
+                flattened.splitSelfTouchingOutlines();
                 flattened.Fracture();
+            }
             else if( aSimplify )
                 flattened.Simplify();
 
@@ -3260,6 +3262,7 @@ void SHAPE_POLY_SET::cacheTriangulation( bool aPartition, bool aSimplify,
         SHAPE_POLY_SET tmpSet( *this );
 
         tmpSet.ClearArcs();
+        tmpSet.splitSelfTouchingOutlines();
         tmpSet.Fracture();
 
         if( !triangulate( tmpSet, -1, m_triangulatedPolys, aHintData ) )

qa/tests/libs/kimath/geometry/test_poly_triangulation.cpp

@@ -505,29 +505,14 @@ BOOST_AUTO_TEST_CASE( Issue18083_SelfIntersectingPolygonArea )
             triangulatedArea += std::abs( tri.Area() );
     }
 
-    // Calculate expected area by simplifying the polygon first.
-    // The simplification should split the self-touching polygon into two separate triangles.
-    SHAPE_POLY_SET simplified;
-    simplified.AddOutline( outline );
-    simplified.Simplify();
-
-    // After simplification, we should have two separate triangular outlines
-    BOOST_TEST( simplified.OutlineCount() == 2 );
-
-    double expectedArea = 0.0;
-
-    for( int ii = 0; ii < simplified.OutlineCount(); ++ii )
-    {
-        expectedArea += std::abs( simplified.Outline( ii ).Area() );
-    }
-
     // The expected total area is 49 mm² (14 mm² + 35 mm² for the two triangular lobes)
+    // Triangle 1: (165,87) - (169,87) - (167,94) = base 4mm, height 7mm = 14 mm²
+    // Triangle 2: (169,87) - (179,87) - (174,94) = base 10mm, height 7mm = 35 mm²
     double expectedAreaMmSq = 49.0 * SCALE * SCALE;
-    BOOST_TEST( std::abs( expectedArea - expectedAreaMmSq ) < expectedAreaMmSq * 0.01 );
 
     // The triangulated area should match the expected area
-    BOOST_TEST( std::abs( triangulatedArea - expectedArea ) < expectedArea * 0.01,
-                "Triangulated area should match simplified polygon area" );
+    BOOST_TEST( std::abs( triangulatedArea - expectedAreaMmSq ) < expectedAreaMmSq * 0.01,
+                "Triangulated area should match expected area of 49 mm²" );
 }
 
 BOOST_AUTO_TEST_CASE( NearlyCollinearVertices )