Fix T58447: Unselected bone used for constraint target

source/blender/blenkernel/BKE_action.h

@@ -160,6 +160,7 @@ void BKE_pose_copy_data(struct bPose **dst, const struct bPose *src, const bool
 void BKE_pose_channel_copy_data(struct bPoseChannel *pchan, const struct bPoseChannel *pchan_from);
 struct bPoseChannel *BKE_pose_channel_find_name(const struct bPose *pose, const char *name);
 struct bPoseChannel *BKE_pose_channel_active(struct Object *ob);
+struct bPoseChannel *BKE_pose_channel_active_or_first_selected(struct Object *ob);
 struct bPoseChannel *BKE_pose_channel_verify(struct bPose *pose, const char *name);
 struct bPoseChannel *BKE_pose_channel_get_mirrored(const struct bPose *pose, const char *name);
 

source/blender/blenkernel/intern/action.c

@@ -43,6 +43,7 @@
 #include "BLT_translation.h"
 
 #include "BKE_action.h"
+#include "BKE_armature.h"
 #include "BKE_anim.h"
 #include "BKE_animsys.h"
 #include "BKE_constraint.h"
@@ -524,6 +525,38 @@ bPoseChannel *BKE_pose_channel_active(Object *ob)
   return NULL;
 }
 
+/**
+ * Use this when detecting the "other selected bone",
+ * when we have multiple armatures in pose mode.
+ *
+ * In this case the active-selected is an obvious choice when finding the target for a
+ * constraint for eg. however from the users perspective the active pose bone of the
+ * active object is the _real_ active bone, so any other non-active selected bone
+ * is a candidate for being the other selected bone, see: T58447.
+ */
+bPoseChannel *BKE_pose_channel_active_or_first_selected(struct Object *ob)
+{
+  bArmature *arm = (ob) ? ob->data : NULL;
+
+  if (ELEM(NULL, ob, ob->pose, arm)) {
+    return NULL;
+  }
+
+  bPoseChannel *pchan = BKE_pose_channel_active(ob);
+  if (pchan && (pchan->bone->flag & BONE_SELECTED) && PBONE_VISIBLE(arm, pchan->bone)) {
+    return pchan;
+  }
+
+  for (pchan = ob->pose->chanbase.first; pchan; pchan = pchan->next) {
+    if (pchan->bone != NULL) {
+      if ((pchan->bone->flag & BONE_SELECTED) && PBONE_VISIBLE(arm, pchan->bone)) {
+        return pchan;
+      }
+    }
+  }
+  return NULL;
+}
+
 /**
  * \see #ED_armature_ebone_get_mirrored (edit-mode, matching function)
  */

source/blender/editors/object/object_constraint.c

@@ -34,6 +34,7 @@
 #include "BLT_translation.h"
 
 #include "DNA_anim_types.h"
+#include "DNA_armature_types.h"
 #include "DNA_constraint_types.h"
 #include "DNA_curve_types.h"
 #include "DNA_scene_types.h"
@@ -1815,10 +1816,15 @@ static bool get_new_constraint_target(
          */
         if ((ob->type == OB_ARMATURE) && (ob->mode & OB_MODE_POSE) &&
             (!only_curve && !only_mesh)) {
-          /* just use the active bone, and assume that it is visible + usable */
-          *tar_ob = ob;
-          *tar_pchan = BKE_pose_channel_active(ob);
-          found = true;
+
+          /* Only use the object & bone if the bone is visible & selected
+           * since we may have multiple objects in pose mode at once.  */
+          bPoseChannel *pchan = BKE_pose_channel_active_or_first_selected(ob);
+          if (pchan != NULL) {
+            *tar_pchan = pchan;
+            *tar_ob = ob;
+            found = true;
+          }
 
           break;
         }