[Tree] Improve navigation with row select mode

Allows navigating with the `ui_left/right` actions when selecting rows,
fixing a navigation inconsistency

scene/gui/tree.cpp

@@ -3468,29 +3468,37 @@ void Tree::gui_input(const Ref<InputEvent> &p_event) {
 			accept_event();
 		}
 
-		if (!selected_item || select_mode == SELECT_ROW || selected_col > (columns.size() - 1)) {
+		if (!selected_item || selected_col > (columns.size() - 1)) {
 			return;
 		}
+
 		if (k.is_valid() && k->is_shift_pressed()) {
 			selected_item->set_collapsed_recursive(false);
-		} else {
+		} else if (select_mode != SELECT_ROW) {
 			_go_right();
+		} else if (selected_item->get_first_child() != nullptr && selected_item->is_collapsed()) {
+			selected_item->set_collapsed(false);
+		} else {
+			_go_down();
 		}
 	} else if (p_event->is_action("ui_left") && p_event->is_pressed()) {
 		if (!cursor_can_exit_tree) {
 			accept_event();
 		}
 
-		if (!selected_item || select_mode == SELECT_ROW || selected_col < 0) {
+		if (!selected_item || selected_col < 0) {
 			return;
 		}
 
 		if (k.is_valid() && k->is_shift_pressed()) {
 			selected_item->set_collapsed_recursive(true);
-		} else {
+		} else if (select_mode != SELECT_ROW) {
 			_go_left();
+		} else if (selected_item->get_first_child() != nullptr && !selected_item->is_collapsed()) {
+			selected_item->set_collapsed(true);
+		} else {
+			_go_up();
 		}
-
 	} else if (p_event->is_action("ui_up") && p_event->is_pressed() && !is_command) {
 		if (!cursor_can_exit_tree) {
 			accept_event();