Cut holes out of offset polygons rather than return separate contours

src/polygons.jl

@@ -1540,7 +1540,20 @@ function _offset(
         add_path!(c, s0.p, j, e)
     end
     result = Clipper.execute(c, Float64(ustrip(delta))) #TODO: fix in clipper
-    return [Polygon(reinterpret(Point{T}, p)) for p in result]
+    polys = [Polygon(reinterpret(Point{T}, p)) for p in result]
+    # Fast path: single or zero contours cannot contain holes
+    if length(polys) <= 1
+        return polys
+    end
+    # Check whether Clipper returned mixed orientations (holes among outer contours).
+    # CW contours are holes in Clipper's convention; CCW are outer boundaries.
+    orientations = [orientation(p) for p in polys]
+    if all(==(first(orientations)), orientations)
+        return polys  # All same orientation — no holes
+    end
+    # Mixed orientations mean holes are present — recombine via union2d so that the
+    # PolyTree keeps hole topology, then flatten back to Polygons with interior cuts.
+    return to_polygons(union2d(polys))
 end
 
 ### cutting algorithm

test/test_clipping.jl

@@ -779,6 +779,35 @@ end
         poly = to_polygons(intersect2d(r0, pa => :test))
         @test length(poly) == 12
     end
+
+    @testset "Offset preserves holes (#11)" begin
+        # Create a ring shape: outer square with inner hole
+        outer = Rectangle(100nm, 100nm)
+        inner = Rectangle(60nm, 60nm) + Point(20nm, 20nm)
+        ring = difference2d(outer, inner)
+
+        # Offset outward — should produce a single polygon (with interior cut
+        # encoding the hole), not two separate flat polygons.
+        # Without the fix, Clipper returns 2 separate Polygons (one for the
+        # expanded outer contour, one for the shrunk inner hole contour) because
+        # _offset loses hole topology.  With the fix, mixed-orientation contours
+        # are recombined via union2d and flattened with interior cuts.
+        result = offset(ring, 5nm)
+        @test length(result) == 1  # single ring polygon with interior cut
+
+        # The result should be larger than the original outer rectangle
+        # (offset expands outward by 5nm on each side)
+        b = bounds(result[1])
+        @test width(b) > 100nm
+        @test height(b) > 100nm
+
+        # Also test with unitless integers (same bug path)
+        outer_i = Rectangle(100, 100)
+        inner_i = Rectangle(60, 60) + Point(20, 20)
+        ring_i = difference2d(outer_i, inner_i)
+        result_i = offset(ring_i, 5)
+        @test length(result_i) == 1
+    end
 end
 
 @testitem "Clipping CurvilinearPolygon" setup = [CommonTestSetup] begin

test/test_render.jl

@@ -1001,8 +1001,10 @@ end
             atol=1e-9μm
         )
         # Halo uses original ClippedPolygon, hole in the center
-        # Offset returns holes as reversed-orientation polygons [issue #11]
-        @test Polygons.orientation(halo(dr1, 0.1μm)[2]) == -1 # still has a hole
+        # Offset preserves holes as interior cuts in keyhole polygons [issue #11 fix]
+        h = halo(dr1, 0.1μm)
+        @test length(h) == 1  # single keyhole polygon (hole encoded as interior cut)
+        @test length(h[1].p) > 4  # more vertices than a simple rectangle = hole present
         @test footprint(union2d(r1, r1 + Point(40, 0)μm)) isa Rectangle # multipolygon => use bounds
         @test halo(union2d(r3), 1μm, -0.5μm) == dr1 # ClippedPolygon halo with inner delta
     end