Fix unstable contours from triangulated planar faces (#15313)

src/libslic3r/TriangleMeshSlicer.cpp

@@ -1461,6 +1461,13 @@ static Polygons make_loops(
     chain_open_polylines_close_gaps(open_polylines, loops, max_gap, true);
 #endif
 
+    // 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(loops);
+
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
     {
         static int iRun = 0;