7 files changed, 73 insertions, 3 deletions
diff --git a/source/blender/blenkernel/BKE_modifier.h b/source/blender/blenkernel/BKE_modifier.h
index 2491f0953c0..b0a7d89e3d8 100644
--- a/source/blender/blenkernel/BKE_modifier.h
+++ b/source/blender/blenkernel/BKE_modifier.h
@@ -407,6 +407,7 @@ struct ModifierData *BKE_modifier_new(int type);
 
 void BKE_modifier_free_ex(struct ModifierData *md, const int flag);
 void BKE_modifier_free(struct ModifierData *md);
+void BKE_modifier_remove_from_list(struct Object *ob, struct ModifierData *md);
 
 /* Generate new UUID for the given modifier. */
 void BKE_modifier_session_uuid_generate(struct ModifierData *md);
diff --git a/source/blender/blenkernel/BKE_object.h b/source/blender/blenkernel/BKE_object.h
index 28c912a4c9a..a620d9af946 100644
--- a/source/blender/blenkernel/BKE_object.h
+++ b/source/blender/blenkernel/BKE_object.h
@@ -77,6 +77,10 @@ bool BKE_object_shaderfx_use_time(struct Object *ob, struct ShaderFxData *md);
 
 bool BKE_object_support_modifier_type_check(const struct Object *ob, int modifier_type);
 
+/* Active modifier. */
+void BKE_object_modifier_set_active(struct Object *ob, struct ModifierData *md);
+struct ModifierData *BKE_object_active_modifier(const struct Object *ob);
+
 bool BKE_object_copy_modifier(struct Object *ob_dst,
                               const struct Object *ob_src,
                               struct ModifierData *md);
diff --git a/source/blender/blenkernel/BKE_screen.h b/source/blender/blenkernel/BKE_screen.h
index 68c341692c2..dea9884f508 100644
--- a/source/blender/blenkernel/BKE_screen.h
+++ b/source/blender/blenkernel/BKE_screen.h
@@ -251,6 +251,11 @@ typedef struct PanelType {
   /* draw entirely, view changes should be handled here */
   void (*draw)(const struct bContext *C, struct Panel *panel);
 
+  /**
+   * Identifier of a boolean property of the panel custom data. Used to draw a highlighted border.
+   */
+  const char *active_property;
+
   /* For instanced panels corresponding to a list: */
 
   /** Reorder function, called when drag and drop finishes. */
diff --git a/source/blender/blenkernel/intern/fluid.c b/source/blender/blenkernel/intern/fluid.c
index 73ca490c395..1bdc2a5dfd2 100644
--- a/source/blender/blenkernel/intern/fluid.c
+++ b/source/blender/blenkernel/intern/fluid.c
@@ -4559,7 +4559,7 @@ void BKE_fluid_particle_system_destroy(struct Object *ob, const int particle_typ
     if (psys->part->type == particle_type) {
       /* clear modifier */
       pfmd = psys_get_modifier(ob, psys);
-      BLI_remlink(&ob->modifiers, (ModifierData *)pfmd);
+      BKE_modifier_remove_from_list(ob, (ModifierData *)pfmd);
       BKE_modifier_free((ModifierData *)pfmd);
 
       /* clear particle system */
diff --git a/source/blender/blenkernel/intern/modifier.c b/source/blender/blenkernel/intern/modifier.c
index 3c8b685a0e0..c9bdaecfa2a 100644
--- a/source/blender/blenkernel/intern/modifier.c
+++ b/source/blender/blenkernel/intern/modifier.c
@@ -212,6 +212,26 @@ void BKE_modifier_free(ModifierData *md)
   BKE_modifier_free_ex(md, 0);
 }
 
+/**
+ * Use instead of `BLI_remlink` when the object's active modifier should change.
+ */
+void BKE_modifier_remove_from_list(Object *ob, ModifierData *md)
+{
+  BLI_assert(BLI_findindex(&ob->modifiers, md) != -1);
+
+  if (md->flag & eModifierFlag_Active) {
+    /* Prefer the previous modifier but use the next if this modifier is the first in the list. */
+    if (md->next != NULL) {
+      BKE_object_modifier_set_active(ob, md->next);
+    }
+    else if (md->prev != NULL) {
+      BKE_object_modifier_set_active(ob, md->prev);
+    }
+  }
+
+  BLI_remlink(&ob->modifiers, md);
+}
+
 void BKE_modifier_session_uuid_generate(ModifierData *md)
 {
   md->session_uuid = BLI_session_uuid_generate();
diff --git a/source/blender/blenkernel/intern/object.c b/source/blender/blenkernel/intern/object.c
index 31d2e3738f4..d747da2213e 100644
--- a/source/blender/blenkernel/intern/object.c
+++ b/source/blender/blenkernel/intern/object.c
@@ -1274,6 +1274,46 @@ void BKE_object_modifier_gpencil_hook_reset(Object *ob, HookGpencilModifierData
   }
 }
 
+/**
+ * Set the object's active modifier.
+ *
+ * \param md: If NULL, only clear the active modifier, otherwise
+ * it must be in the #Object.modifiers list.
+ */
+void BKE_object_modifier_set_active(Object *ob, ModifierData *md)
+{
+  LISTBASE_FOREACH (ModifierData *, md_iter, &ob->modifiers) {
+    md_iter->flag &= ~eModifierFlag_Active;
+  }
+
+  if (md != NULL) {
+    BLI_assert(BLI_findindex(&ob->modifiers, md) != -1);
+    md->flag |= eModifierFlag_Active;
+  }
+}
+
+ModifierData *BKE_object_active_modifier(const Object *ob)
+{
+  /* In debug mode, check for only one active modifier. */
+#ifndef NDEBUG
+  int active_count = 0;
+  LISTBASE_FOREACH (ModifierData *, md, &ob->modifiers) {
+    if (md->flag & eModifierFlag_Active) {
+      active_count++;
+    }
+  }
+  BLI_assert(ELEM(active_count, 0, 1));
+#endif
+
+  LISTBASE_FOREACH (ModifierData *, md, &ob->modifiers) {
+    if (md->flag & eModifierFlag_Active) {
+      return md;
+    }
+  }
+
+  return NULL;
+}
+
 bool BKE_object_support_modifier_type_check(const Object *ob, int modifier_type)
 {
   const ModifierTypeInfo *mti;
diff --git a/source/blender/blenkernel/intern/particle.c b/source/blender/blenkernel/intern/particle.c
index 0b331fb88d2..d516de535f9 100644
--- a/source/blender/blenkernel/intern/particle.c
+++ b/source/blender/blenkernel/intern/particle.c
@@ -4046,7 +4046,7 @@ void object_remove_particle_system(Main *bmain, Scene *UNUSED(scene), Object *ob
   /* Clear modifier, skip empty ones. */
   psmd = psys_get_modifier(ob, psys);
   if (psmd) {
-    BLI_remlink(&ob->modifiers, psmd);
+    BKE_modifier_remove_from_list(ob, (ModifierData *)psmd);
     BKE_modifier_free((ModifierData *)psmd);
   }
 
@@ -5401,7 +5401,7 @@ void BKE_particle_system_blend_read_lib(BlendLibReader *reader,
     else {
       /* particle modifier must be removed before particle system */
       ParticleSystemModifierData *psmd = psys_get_modifier(ob, psys);
-      BLI_remlink(&ob->modifiers, psmd);
+      BKE_modifier_remove_from_list(ob, (ModifierData *)psmd);
       BKE_modifier_free((ModifierData *)psmd);
 
       BLI_remlink(particles, psys);