Call update_scrollbars() in sort_children() so the max is set properly so you can set scroll_container.scroll_vertical instantly after adding children to a scroll container.

scene/gui/scroll_container.cpp

@@ -312,6 +312,7 @@ void ScrollContainer::_update_dimensions() {
 		fit_child_in_rect(c, r);
 	}
 
+	update_scrollbars();
 	update();
 }