fix: regression: couldn't edit color or gradient of multiple layers (synfig#3470)

Introduced when fixing synfig#3288 ( 483b5ed - PR 3373 ).

By the way, both commits fix another case:

(In Synfig <= 1.5.3)
1. Create a text layer
2. Create a star layer
3. Select the text layer
4. Click on color parameter of text layer to open Color editor
5. Select both layers (text and star)
6. Change the color in the previously opened Color editor
7. It changes the text color, but yells an error to star

The problem does not happen if you open color dialog after select
both layers (step #5 above)

fix synfig#3469

Author: rodolforg

synfig-studio/src/gui/cellrenderer/cellrenderer_value.cpp

@@ -574,58 +574,95 @@ void
 CellRenderer_ValueBase::gradient_edited(synfig::Gradient gradient, Glib::ustring path)
 {
 	synfigapp::ValueDesc vd = App::dialog_gradient->get_value_desc();
-	if (vd.is_valid()) {
-		if (vd.parent_is_layer()) {
-			if (is_layer_deleted(vd.get_layer()))
-				return;
-		}
-		// do not emit signal_edited_ : gradient dialog may be opened by other cell
-		// and would try to update the parameter related to current one
-		// as it just remembered its parameter tree path
-		// Example:
-		// 1. Create a region layer
-		// 2. Create a gradient layer
-		// 3. Click on gradient parameter to edit it
-		// 4. Without closing gradient dialog, select the region layer instead
-		// 5. Try to edit the gradient
-		// 6. Error Message appears
+	if (!vd.is_valid()) {
+		ValueBase old_value(property_value_.get_value());
+		ValueBase value(gradient);
+		if (old_value != value)
+			signal_edited_(path, value);
+
+		return;
+	}
+
+	if (!vd.parent_is_layer()) {
 		canvas_interface->change_value(vd, gradient);
 		return;
 	}
 
-	ValueBase old_value(property_value_.get_value());
-	ValueBase value(gradient);
-	if (old_value != value)
-		signal_edited_(path, value);
+	if (is_layer_deleted(vd.get_layer()))
+		return;
+
+	// do not emit signal_edited_ : gradient dialog may be opened by other cell
+	// and would try to update the parameter related to current one
+	// as it just remembered its parameter tree path
+	// Example:
+	// 1. Create a region layer
+	// 2. Create a gradient layer
+	// 3. Click on gradient parameter to edit it
+	// 4. Without closing gradient dialog, select the region layer instead
+	// 5. Try to edit the gradient
+	// 6. Error Message appears
+	synfigapp::SelectionManager::LayerList selected_layers = canvas_interface->get_selection_manager()->get_selected_layers();
+	const Layer::Handle& trigger_layer = vd.get_layer();
+	bool is_trigger_layer_still_selected = std::find(selected_layers.begin(), selected_layers.end(), trigger_layer) != selected_layers.end();
+	if (!is_trigger_layer_still_selected)
+		return;
+
+	if (selected_layers.size() <= 1) {
+		canvas_interface->change_value(vd, gradient);
+	} else {
+		synfigapp::Action::PassiveGrouper set_multiple_color_action(canvas_interface->get_instance().get(), strprintf(_("Set Layer Gradients (%d)"), selected_layers.size()));
+		for (const auto& layer : selected_layers) {
+			canvas_interface->change_value(synfigapp::ValueDesc(layer, vd.get_param_name()), gradient);
+		}
+	}
 }
 
 void
 CellRenderer_ValueBase::color_edited(synfig::Color color, Glib::ustring path)
 {
 	synfigapp::ValueDesc vd = App::dialog_color->get_value_desc();
-	if (vd.is_valid()) {
-		if (vd.parent_is_layer()) {
-			if (is_layer_deleted(vd.get_layer()))
-				return;
-		}
-		// do not emit signal_edited_ : color dialog may be opened by other cell
-		// and would try to update the parameter related to current one
-		// as it just remembered its parameter tree path
-		// Example:
-		// 1. Create a region layer
-		// 2. Create an outline layer
-		// 3. Click on color parameter to edit it
-		// 4. Without closing color dialog, select the region layer instead
-		// 5. Try to edit the color
-		// 6. Error Message appears
+	if (!vd.is_valid()) {
+		ValueBase old_value(property_value_.get_value());
+		ValueBase value(color);
+		if (old_value != value)
+			signal_edited_(path, value);
+
+		return;
+	}
+
+	if (!vd.parent_is_layer()) {
 		canvas_interface->change_value(vd, color);
 		return;
 	}
 
-	ValueBase old_value(property_value_.get_value());
-	ValueBase value(color);
-	if (old_value != value)
-		signal_edited_(path, value);
+	if (is_layer_deleted(vd.get_layer()))
+		return;
+
+	// do not emit signal_edited_ : color dialog may be opened by other cell
+	// and would try to update the parameter related to current one
+	// as it just remembered its parameter tree path
+	// Example:
+	// 1. Create a region layer
+	// 2. Create an outline layer
+	// 3. Click on color parameter to edit it
+	// 4. Without closing color dialog, select the region layer instead
+	// 5. Try to edit the color
+	// 6. Error Message appears
+
+	synfigapp::SelectionManager::LayerList selected_layers = canvas_interface->get_selection_manager()->get_selected_layers();
+	const Layer::Handle& trigger_layer = vd.get_layer();
+	bool is_trigger_layer_still_selected = std::find(selected_layers.begin(), selected_layers.end(), trigger_layer) != selected_layers.end();
+	if (!is_trigger_layer_still_selected)
+		return;
+
+	if (selected_layers.size() <= 1) {
+		canvas_interface->change_value(vd, color);
+	} else {
+		synfigapp::Action::PassiveGrouper set_multiple_color_action(canvas_interface->get_instance().get(), strprintf(_("Set Layer Colors (%d)"), selected_layers.size()));
+		for (const auto& layer : selected_layers) {
+			canvas_interface->change_value(synfigapp::ValueDesc(layer, vd.get_param_name()), color);
+		}
+	}
 }
 
 Gtk::CellEditable*