Anim: gray out disconnected action slots in channel list

The approach taken here is really simple: just check if the channel
either is or belongs to an action slot, and if so then reduce the
alpha of its text label if it's unselected and its slot has no users.

Pull Request: https://projects.blender.org/blender/blender/pulls/127257

source/blender/editors/animation/anim_channels_defines.cc

@@ -4821,6 +4821,50 @@ static bool achannel_is_being_renamed(const bAnimContext *ac,
   return false;
 }
 
+/**
+ * Check if the animation channel is an item that either is or belongs to a
+ * disconnected action slot.
+ */
+static bool achannel_is_part_of_disconnected_slot(const bAnimListElem *ale)
+{
+  BLI_assert(ale->bmain != nullptr);
+  if (ale->bmain == nullptr) {
+    return false;
+  }
+
+  switch (ale->type) {
+    case ANIMTYPE_ACTION_SLOT: {
+      const animrig::Slot &slot = static_cast<const ActionSlot *>(ale->data)->wrap();
+
+      return slot.users(*ale->bmain).is_empty();
+    }
+
+    case ANIMTYPE_GROUP:
+    case ANIMTYPE_FCURVE: {
+      if (ale->fcurve_owner_id == nullptr || GS(ale->fcurve_owner_id->name) != ID_AC) {
+        return false;
+      }
+
+      const animrig::Action &action =
+          reinterpret_cast<const bAction *>(ale->fcurve_owner_id)->wrap();
+      if (action.is_action_legacy()) {
+        return false;
+      }
+
+      const animrig::Slot *slot = action.slot_for_handle(ale->slot_handle);
+      if (slot == nullptr) {
+        return false;
+      }
+
+      return slot->users(*ale->bmain).is_empty();
+    }
+
+    /* No other types are currently drawn as children of action slots. */
+    default:
+      return false;
+  }
+}
+
 /** Check if the animation channel name should be underlined in red due to errors. */
 static bool achannel_is_broken(const bAnimListElem *ale)
 {
@@ -5014,6 +5058,11 @@ void ANIM_channel_draw(
       UI_GetThemeColor4ubv(TH_TEXT, col);
     }
 
+    /* Gray out disconnected action slots and their children. */
+    if (!selected && achannel_is_part_of_disconnected_slot(ale)) {
+      col[3] = col[3] / 3 * 2;
+    }
+
     /* get name */
     acf->name(ale, name);