Don't attempt to insert points into degenerate triangles.

Use a simpler degenerate triangle check that calculates area.

modules/csg/csg.cpp

@@ -154,6 +154,14 @@ inline bool is_point_in_triangle(const Vector3 &p_point, const Vector3 p_vertice
 	return true;
 }
 
+inline static bool is_triangle_degenerate(const Vector2 p_vertices[3], real_t p_vertex_snap2) {
+	real_t det = p_vertices[0].x * p_vertices[1].y - p_vertices[0].x * p_vertices[2].y +
+				 p_vertices[0].y * p_vertices[2].x - p_vertices[0].y * p_vertices[1].x +
+				 p_vertices[1].x * p_vertices[2].y - p_vertices[1].y * p_vertices[2].x;
+
+	return det < p_vertex_snap2;
+}
+
 inline static bool are_segements_parallel(const Vector2 p_segment1_points[2], const Vector2 p_segment2_points[2], float p_vertex_snap2) {
 	Vector2 segment1 = p_segment1_points[1] - p_segment1_points[0];
 	Vector2 segment2 = p_segment2_points[1] - p_segment2_points[0];
@@ -1117,6 +1125,11 @@ int CSGBrushOperation::Build2DFaces::_insert_point(const Vector2 &p_point) {
 			face_vertices[2].uv
 		};
 
+		// Skip degenerate triangles.
+		if (is_triangle_degenerate(points, vertex_snap2)) {
+			continue;
+		}
+
 		// Check if point is existing face vertex.
 		for (int i = 0; i < 3; ++i) {
 			if ((p_point - face_vertices[i].point).length_squared() < vertex_snap2) {
@@ -1198,11 +1211,8 @@ int CSGBrushOperation::Build2DFaces::_insert_point(const Vector2 &p_point) {
 			// The new vertex is the last vertex.
 			for (int i = 0; i < 3; ++i) {
 				// Don't create degenerate triangles.
-				Vector2 edge[2] = { points[i], points[(i + 1) % 3] };
-				Vector2 new_edge1[2] = { vertices[new_vertex_idx].point, points[i] };
-				Vector2 new_edge2[2] = { vertices[new_vertex_idx].point, points[(i + 1) % 3] };
-				if (are_segements_parallel(edge, new_edge1, vertex_snap2) &&
-						are_segements_parallel(edge, new_edge2, vertex_snap2)) {
+				Vector2 new_points[3] = { points[i], points[(i + 1) % 3], vertices[new_vertex_idx].point };
+				if (is_triangle_degenerate(new_points, vertex_snap2)) {
 					continue;
 				}