diff --git a/notes.md b/notes.md
index 2ab53666..473bf6f7 100644
--- a/notes.md
+++ b/notes.md
@@ -60,6 +60,7 @@
 * `B` outside cutting direction in roughing mode inverted
 * `B` clear voids has double interior pass because wrong offset shadow used
 * `B` top clearing operations should use linear, not offset, passes
+* `F` use 3 registration holes to prevent rotation accidents (midline offset one axis)
 * `F` parameterize dropping close points in prep.js. ensure long segments remain straight
 * `F` flat and volumetric rendering of paths
 * `F` z bounded slices (extension of z bottom offset feature)
diff --git a/src/kiri/slicer.js b/src/kiri/slicer.js
index 12ff1d71..6164b5bf 100644
--- a/src/kiri/slicer.js
+++ b/src/kiri/slicer.js
@@ -688,8 +688,8 @@
                 }
             }
 
+            // undo temp del/used marks
             for (let i=0; i<stack.length; i++) stack[i].del = false;
-            // stack.forEach(function(p) { p.del = false });
         }
 
         // emit a polygon if it can be cleaned and still have 2 or more points
diff --git a/src/mode/fdm/slice.js b/src/mode/fdm/slice.js
index c3864b11..ea6b5474 100644
--- a/src/mode/fdm/slice.js
+++ b/src/mode/fdm/slice.js
@@ -388,6 +388,11 @@
                     last = top_poly.clone(true);
                     top.shells = last;
                 } else {
+                    // heal top open polygons if the ends are close (benchy tilt test)
+                    top_poly.forEach(p => { if (p.open) {
+                        let dist = p.first().distTo2D(p.last());
+                        if (dist < 1) p.open = false;
+                    } });
                     if (opt.thin) {
                         top.thin_fill = [];
                         let oso = {z, count, gaps: [], outs: [], minArea: 0.05};
@@ -913,7 +918,7 @@
 
         // return top.supports = supports;
         // then union supports
-        supports = POLY.union(supports);
+        supports = POLY.union(supports, null, true);
 
         // constrain support poly to top polys
         supports = POLY.trimTo(supports, trimTo);
@@ -963,7 +968,7 @@
         if (!supports) return;
 
         // union supports
-        supports = POLY.union(supports);
+        supports = POLY.union(supports, undefined, true);
 
         // trim to clip offsets
         if (slice.offsets) POLY.subtract(supports, slice.offsets, nsB, null, slice.z, min);