1 files changed, 35 insertions, 15 deletions

diff --git a/source/blender/blenkernel/intern/mesh_mirror.c b/source/blender/blenkernel/intern/mesh_mirror.c
index 6df13e71e72..2d4308945fc 100644
--- a/source/blender/blenkernel/intern/mesh_mirror.c
+++ b/source/blender/blenkernel/intern/mesh_mirror.c
@@ -130,14 +130,11 @@ void BKE_mesh_mirror_apply_mirror_on_axis(struct Main *bmain,
   BM_mesh_free(bm);
 }
 
-/**
- * \warning This should _not_ be used to modify original meshes since
- * it doesn't handle shape-keys, use #BKE_mesh_mirror_apply_mirror_on_axis instead.
- */
 Mesh *BKE_mesh_mirror_apply_mirror_on_axis_for_modifier(MirrorModifierData *mmd,
                                                         Object *ob,
                                                         const Mesh *mesh,
-                                                        const int axis)
+                                                        const int axis,
+                                                        const bool use_correct_order_on_merge)
 {
   const float tolerance_sq = mmd->tolerance * mmd->tolerance;
   const bool do_vtargetmap = (mmd->flag & MOD_MIR_NO_MERGE) == 0;
@@ -266,23 +263,46 @@ Mesh *BKE_mesh_mirror_apply_mirror_on_axis_for_modifier(MirrorModifierData *mmd,
        * Always merge from the copied into the original vertices so it's possible to
        * generate a 1:1 mapping by scanning vertices from the beginning of the array
        * as is done in #BKE_editmesh_vert_coords_when_deformed. Without this,
-       * the coordinates returned will sometimes point to the copied vertex locations, see: T91444.
+       * the coordinates returned will sometimes point to the copied vertex locations, see:
+       * T91444.
+       *
+       * However, such a change also affects non-versionable things like some modifiers binding, so
+       * we cannot enforce that behavior on existing modifiers, in which case we keep using the
+       * old, incorrect behavior of merging the source vertex into its copy.
        */
-      if (UNLIKELY(len_squared_v3v3(mv_prev->co, mv->co) < tolerance_sq)) {
-        *vtmap_b = i;
-        tot_vtargetmap++;
+      if (use_correct_order_on_merge) {
+        if (UNLIKELY(len_squared_v3v3(mv_prev->co, mv->co) < tolerance_sq)) {
+          *vtmap_b = i;
+          tot_vtargetmap++;
+
+          /* average location */
+          mid_v3_v3v3(mv->co, mv_prev->co, mv->co);
+          copy_v3_v3(mv_prev->co, mv->co);
+        }
+        else {
+          *vtmap_b = -1;
+        }
 
-        /* average location */
-        mid_v3_v3v3(mv->co, mv_prev->co, mv->co);
-        copy_v3_v3(mv_prev->co, mv->co);
+        /* Fill here to avoid 2x loops. */
+        *vtmap_a = -1;
       }
       else {
+        if (UNLIKELY(len_squared_v3v3(mv_prev->co, mv->co) < tolerance_sq)) {
+          *vtmap_a = maxVerts + i;
+          tot_vtargetmap++;
+
+          /* average location */
+          mid_v3_v3v3(mv->co, mv_prev->co, mv->co);
+          copy_v3_v3(mv_prev->co, mv->co);
+        }
+        else {
+          *vtmap_a = -1;
+        }
+
+        /* Fill here to avoid 2x loops. */
         *vtmap_b = -1;
       }
 
-      /* Fill here to avoid 2x loops. */
-      *vtmap_a = -1;
-
       vtmap_a++;
       vtmap_b++;
     }