From f05444dc94bc325a4eef1ec1dafc33e1331caec9 Mon Sep 17 00:00:00 2001
From: Valerii Bokhan <80919135+valerii-bokhan@users.noreply.github.com>
Date: Fri, 21 Aug 2026 15:50:58 +0200
Subject: [PATCH] Fix unstable contours from triangulated planar faces 
 (redone) (#15316)

---
 src/libslic3r/TriangleMeshSlicer.cpp | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/libslic3r/TriangleMeshSlicer.cpp b/src/libslic3r/TriangleMeshSlicer.cpp
index 2c1c0da23f..c403a6bd92 100644
--- a/src/libslic3r/TriangleMeshSlicer.cpp
+++ b/src/libslic3r/TriangleMeshSlicer.cpp
@@ -1499,6 +1499,13 @@ static std::vector<Polygons> make_loops(
                 Polygons &polygons = layers[line_idx];
                 polygons = make_loops(lines[line_idx]);
 
+                // Orca: A planar quad represented by two triangles contributes a point where the
+                // slicing plane crosses the shared diagonal. After rounding to coord_t this
+                // point may be very slightly off the otherwise straight contour edge. Apart
+                // from being redundant, such points make the subsequent contour
+                // simplification depend on the slice height (and may move seam candidates).
+                remove_collinear(polygons);
+
                 auto this_mode = line_idx < params.slicing_mode_normal_below_layer ? params.mode_below : params.mode;
                 if (! polygons.empty()) {
                     if (this_mode == MeshSlicingParams::SlicingMode::Positive) {