There's a UISystem now for to compliment the entities and components in GUECS. I now use that to do the drag/drop transfers instead of raw code right in the FSM.

gui/uisystems.cpp

@@ -0,0 +1,25 @@
+#include "gui/uisystems.hpp"
+#include "gui/guecstra.hpp"
+
+namespace UISystem {
+  std::optional<guecs::Entity> loot_grab(guecs::UI& gui, guecs::Entity gui_id) {
+    if(auto source = gui.get_if<guecs::GrabSource>(gui_id)) {
+      source->grab();
+      return gui_id;
+    } else {
+      return std::nullopt;
+    }
+  }
+
+  bool loot_drop(guecs::UI& source, guecs::UI& target, guecs::Entity source_id, guecs::Entity target_id) {
+    auto& drop = target.get<guecs::DropTarget>(target_id);
+    auto& grab = source.get<guecs::GrabSource>(source_id);
+
+    if(drop.commit(grab.world_entity)) {
+      grab.commit();
+      return true;
+    } else {
+      return false;
+    }
+  }
+}