From b42e4052b7b6472c72897a0239f964aea2b81b98 Mon Sep 17 00:00:00 2001
From: Germano Cavalcante <germano.costa@ig.com.br>
Date: Tue, 15 Jul 2025 20:16:47 -0300
Subject: [PATCH] Fix #121378: Can't snap parent to children with Affect Only
 Parent activated

The flag that skips objects for snapping (`BA_SNAP_FIX_DEPS_FIASCO`) is
set for all objects that have a `DEG_OB_COMP_TRANSFORM` relation.

However, when the **Affect Only Parent** option is enabled, children
are not transformed and therefore could still be snapped to.
Despite this, the `DEG_OB_COMP_TRANSFORM` dependency remains present on
them.

To address this, a new callback was introduced that skips
Transform-to-Transform dependencies while preserving
Transform-to-Geometry dependencies in this scenario.

This approach may introduce some false positives, leading to spurious
dependency cycles (e.g., when constraints are involved).
Nevertheless, the trade-off favors accepting these false positives
rather than disabling the feature altogether.

Also make non-functional change that just removes a flag before the snap
loop, thus avoiding checking if the flag exists inside the loop.

Ref !142007
---
 .../transform/transform_convert_object.cc     | 35 +++++++++++++++----
 .../transform/transform_snap_object.cc        |  4 +--
 2 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/source/blender/editors/transform/transform_convert_object.cc b/source/blender/editors/transform/transform_convert_object.cc
index 8a883a11bc2..2f7265c299e 100644
--- a/source/blender/editors/transform/transform_convert_object.cc
+++ b/source/blender/editors/transform/transform_convert_object.cc
@@ -311,7 +311,18 @@ static void trans_object_base_deps_flag_prepare(const TransInfo *t,
   }
 }
 
-static void set_trans_object_base_deps_flag_cb(ID *id, eDepsObjectComponentType component)
+static void tag_trans_objects_with_geometry_dep_only_fn(ID *id, eDepsObjectComponentType component)
+{
+  /* Here we only handle object IDs. */
+  if (GS(id->name) != ID_OB) {
+    return;
+  }
+  if (component == DEG_OB_COMP_GEOMETRY) {
+    id->tag |= ID_TAG_DOIT;
+  }
+}
+
+static void tag_trans_objects_dep_fn(ID *id, eDepsObjectComponentType component)
 {
   /* Here we only handle object IDs. */
   if (GS(id->name) != ID_OB) {
@@ -329,11 +340,21 @@ static void flush_trans_object_base_deps_flag(const TransInfo *t, Object *object
     return;
   }
   object->id.tag |= ID_TAG_DOIT;
-  DEG_foreach_dependent_ID_component(t->depsgraph,
-                                     &object->id,
-                                     DEG_OB_COMP_TRANSFORM,
-                                     DEG_FOREACH_COMPONENT_IGNORE_TRANSFORM_SOLVERS,
-                                     set_trans_object_base_deps_flag_cb);
+
+  DEG_foreach_dependent_ID_component(
+      t->depsgraph,
+      &object->id,
+      DEG_OB_COMP_TRANSFORM,
+      DEG_FOREACH_COMPONENT_IGNORE_TRANSFORM_SOLVERS,
+
+      /* When we transform parents while skipping children, we only traverse the GEOMETRY-dependent
+       * components. This avoids marking children as not participating in snapping but still marks
+       * objects with modifier dependencies.
+       * Unfortunately, some transform-dependent objects that are not children may also be skipped,
+       * such as constrained ones.
+       * See #121378 for details. */
+      (t->options & CTX_OBMODE_XFORM_SKIP_CHILDREN) ? tag_trans_objects_with_geometry_dep_only_fn :
+                                                      tag_trans_objects_dep_fn);
 }
 
 static void trans_object_base_deps_flag_finish(const TransInfo *t,
@@ -690,6 +711,7 @@ static void createTransObject(bContext *C, TransInfo *t)
                       tdo->xcs, ob, ob_parent_recurse, object::XFORM_OB_SKIP_CHILD_PARENT_APPLY);
                   BLI_ghash_insert(objects_parent_root, ob, ob_parent_recurse);
                   base->flag_legacy |= BA_TRANSFORM_LOCKED_IN_PLACE;
+                  base->flag_legacy &= ~BA_SNAP_FIX_DEPS_FIASCO;
                 }
               }
             }
@@ -711,6 +733,7 @@ static void createTransObject(bContext *C, TransInfo *t)
             object::object_xform_skip_child_container_item_ensure(
                 tdo->xcs, ob, nullptr, object::XFORM_OB_SKIP_CHILD_PARENT_IS_XFORM);
             base->flag_legacy |= BA_TRANSFORM_LOCKED_IN_PLACE;
+            base->flag_legacy &= ~BA_SNAP_FIX_DEPS_FIASCO;
           }
           else {
             Object *ob_parent_recurse = static_cast<Object *>(
diff --git a/source/blender/editors/transform/transform_snap_object.cc b/source/blender/editors/transform/transform_snap_object.cc
index 270aee09dac..d726fa279d7 100644
--- a/source/blender/editors/transform/transform_snap_object.cc
+++ b/source/blender/editors/transform/transform_snap_object.cc
@@ -409,9 +409,7 @@ static bool snap_object_is_snappable(const SnapObjectContext *sctx,
     return false;
   }
 
-  if ((snap_target_select == SCE_SNAP_TARGET_ALL) ||
-      (base->flag_legacy & BA_TRANSFORM_LOCKED_IN_PLACE))
-  {
+  if (snap_target_select == SCE_SNAP_TARGET_ALL) {
     return true;
   }