8.4 version has added big update and support for Elementor plugin. However, there are some small changes which can affect sites.
First of all, we added second level for top line menu. You can add there some important and system elements, like language switcher. You can enable top line in theme option – header option or in Customizer
Also, due to constant problem with wordpress sidebar after each update we must to change ID of sidebar to prevent this in future. So, after this update, you will lose sidebar widgets. Anyway, all widgets are safe, you can find them in Inactive widgets in bottom of page. So, just drag and drop them in Sidebar widget area
If you used Visual Composer on pages and Widgetised Sidebar module, you also need to choose proper Area in options of module. So, edit page, click on widget module and choose there Sidebar Area, resave page
Sorry for this problem, unfortunately, wordpress doesn’t allow to change ID of sidebars in code