part: fixes #27365 mirror place copy at correct location (#27370)

src/Mod/Part/App/FeatureMirroring.cpp

@@ -309,12 +309,48 @@ App::DocumentObjectExecReturn* Mirroring::execute()
     Base::Vector3d norm = Normal.getValue();
 
     try {
+        // get shape without transform
+        auto shape = Feature::getTopoShape(link, ShapeOption::ResolveLink);
+
+        // manually apply placement via setPlacement() before mirroring
+        if (link->isDerivedFrom(App::GeoFeature::getClassTypeId())) {
+            App::GeoFeature* geo = static_cast<App::GeoFeature*>(link);
+            Base::Placement placement = geo->Placement.getValue();
+
+            if (!placement.isIdentity()) {
+                // Convert Placement to gp_Trsf
+                gp_Trsf trsf;
+                Base::Matrix4D mat = placement.toMatrix();
+                trsf.SetValues(
+                    mat[0][0],
+                    mat[0][1],
+                    mat[0][2],
+                    mat[0][3],
+                    mat[1][0],
+                    mat[1][1],
+                    mat[1][2],
+                    mat[1][3],
+                    mat[2][0],
+                    mat[2][1],
+                    mat[2][2],
+                    mat[2][3]
+                );
+
+                // actually transform the geometry (copy=true to create new shape)
+                BRepBuilderAPI_Transform mkTrf(shape.getShape(), trsf, Standard_True);
+                shape = TopoShape(mkTrf.Shape());
+            }
+        }
+
         gp_Ax2 ax2(gp_Pnt(base.x, base.y, base.z), gp_Dir(norm.x, norm.y, norm.z));
-        auto shape = Feature::getTopoShape(link, ShapeOption::ResolveLink | ShapeOption::Transform);
+
         if (shape.isNull()) {
             Standard_Failure::Raise("Cannot mirror empty shape");
         }
-        this->Shape.setValue(TopoShape(0).makeElementMirror(shape, ax2));
+
+        auto mirrored = TopoShape(0).makeElementMirror(shape, ax2);
+
+        this->Shape.setValue(mirrored);
         copyMaterial(link);
 
         return Part::Feature::execute();

src/Mod/Part/CMakeLists.txt

@@ -79,6 +79,7 @@ set(Part_tests
     parttests/ColorTransparencyTest.py
     parttests/TopoShapeTest.py
     parttests/TestTangentMode3-0.21.FCStd
+    parttests/TestPartMirror.py
 )
 
 add_custom_target(PartScripts ALL SOURCES

src/Mod/Part/TestPartApp.py

@@ -35,6 +35,7 @@ from parttests.Geom2d_tests import Geom2dTests
 from parttests.regression_tests import RegressionTests
 from parttests.TopoShapeListTest import TopoShapeListTest
 from parttests.TopoShapeTest import TopoShapeTest
+from parttests.TestPartMirror import TestPartMirroringRegression
 
 
 # ---------------------------------------------------------------------------

src/Mod/Part/parttests/TestPartMirror.py

@@ -0,0 +1,100 @@
+"""
+this test will FAIL on current main branch (with PR #26963) and PASS after the fix.
+current main at: https://github.com/FreeCAD/FreeCAD/tree/24f0c8e2c321bb202410feae84eb58779ba8e8d2
+"""
+
+import unittest
+import FreeCAD as App
+import Part
+import Draft
+
+
+class TestPartMirroringRegression(unittest.TestCase):
+    """Regression test for GitHub issue #27365.
+
+    Part::Mirroring with Draft Clone produces incorrect results after PR #26963.
+    The mirror position shifts on recompute when the source has non-identity Placement.
+    """
+
+    def setUp(self):
+        self.doc = App.newDocument("TestMirrorRegression")
+
+    def tearDown(self):
+        App.closeDocument(self.doc.Name)
+
+    def testMirroringWithDraftCloneStability(self):
+        """Test that Part::Mirroring position is stable across recomputes.
+
+        This test reproduces issue #27365: mirroring a Draft Clone that has
+        placement, and causes the mirror position to be incorrect and/or shift
+        on recompute.
+
+        This test FAILS on current main (after PR #26963) and should pass after a fix is applied.
+        """
+        # create sketch at origin
+        sketch = self.doc.addObject("Sketcher::SketchObject", "Sketch")
+        sketch.addGeometry(Part.LineSegment(App.Vector(0, 0, 0), App.Vector(10, 0, 0)), False)
+        sketch.addGeometry(Part.LineSegment(App.Vector(10, 0, 0), App.Vector(10, 10, 0)), False)
+        sketch.addGeometry(Part.LineSegment(App.Vector(10, 10, 0), App.Vector(0, 10, 0)), False)
+        sketch.addGeometry(Part.LineSegment(App.Vector(0, 10, 0), App.Vector(0, 0, 0)), False)
+        self.doc.recompute()
+
+        # create draft clone at X=30 with 90° rotation around Z
+        clone = Draft.make_clone(sketch)
+        clone.Placement = App.Placement(App.Vector(30, 0, 0), App.Rotation(App.Vector(0, 0, 1), 90))
+        self.doc.recompute()
+
+        # verify clone is positioned correctly
+        clone_bbox = clone.Shape.BoundBox
+        clone_center_x = (clone_bbox.XMin + clone_bbox.XMax) / 2
+        self.assertAlmostEqual(
+            clone_center_x,
+            30.0,
+            delta=5.0,
+            msg=f"clone should be centered around X=30, but is at X={clone_center_x:.2f}",
+        )
+
+        # mirror across YZ plane (X=0, normal in +X direction)
+        mirror = self.doc.addObject("Part::Mirroring", "Mirror")
+        mirror.Source = clone
+        mirror.Base = App.Vector(0, 0, 0)
+        mirror.Normal = App.Vector(1, 0, 0)
+        self.doc.recompute()
+
+        # get initial mirror position
+        initial_bbox = mirror.Shape.BoundBox
+        initial_center_x = (initial_bbox.XMin + initial_bbox.XMax) / 2
+
+        # mirror should be on opposite side of plane from clone
+        # clone is at X≈30, plane at X=0, so mirror should be at X≈-30
+        self.assertLess(
+            initial_center_x,
+            -20.0,
+            msg=f"mirror should be at X≈-30 (opposite side of X=0 from clone at X≈30), "
+            f"but is at X={initial_center_x:.2f}",
+        )
+
+        # position must be stable across recompute
+        # this is the core bug, ie. position shifts on recompute
+        for i in range(5):
+            self.doc.recompute()
+
+        final_bbox = mirror.Shape.BoundBox
+        final_center_x = (final_bbox.XMin + final_bbox.XMax) / 2
+
+        # position should not change (within tolerance)
+        self.assertAlmostEqual(
+            initial_center_x,
+            final_center_x,
+            places=3,
+            msg=f"mirror position shifted on recompute: "
+            f"X={initial_center_x:.6f} -> X={final_center_x:.6f}",
+        )
+
+
+# for standalone execution
+if __name__ == "__main__":
+    suite = unittest.TestSuite()
+    suite.addTest(unittest.TestLoader().loadTestsFromTestCase(TestPartMirroringRegression))
+    runner = unittest.TextTestRunner()
+    runner.run(suite)