Improved copying the data from an action to an another. I'll try an another solution though.

modules/paint/actions/paint_action.cpp

@@ -181,6 +181,25 @@ void PaintAction::draw_points(PaintCanvas *canvas, const PoolVector2iArray &poin
 	}
 }
 
+void PaintAction::copy_data_from(const Ref<PaintAction> &other) {
+	action_data_undo = other->action_data_undo.duplicate(true);
+	action_data_redo = other->action_data_redo.duplicate(true);
+	action_data_preview = other->action_data_preview.duplicate(true);
+
+	undo_cells.append_array(other->undo_cells);
+	undo_colors.append_array(other->undo_colors);
+
+	redo_cells.append_array(other->redo_cells);
+	redo_colors.append_array(other->redo_colors);
+
+	preview_cells.append_array(other->preview_cells);
+	preview_colors.append_array(other->preview_colors);
+
+	layer = other->layer;
+
+	action_data = other->action_data.duplicate(true);
+}
+
 PaintAction::PaintAction() {
 }
 

modules/paint/actions/paint_action.h

@@ -56,6 +56,8 @@ public:
 
 	void draw_points(PaintCanvas *canvas, const PoolVector2iArray &point_arr, const PoolColorArray &color_arr);
 
+	void copy_data_from(const Ref<PaintAction> &other);
+
 	PaintAction();
 	~PaintAction();
 

modules/paint/paint_window.cpp

@@ -609,7 +609,8 @@ void PaintWindow::commit_action() {
 	//print("commit action")
 	_current_action->commit_action(paint_canvas);
 	Ref<PaintAction> action = get_action();
-	action->action_data = _current_action->action_data.duplicate(true);
+	action->copy_data_from(_current_action);
+
 	_actions_history.push_back(action);
 	_redo_history.clear();