I think at the very least individual widgets should wait to be fully initialized before becoming interactable. The amount of times I've, say, tried to click on a dropdown menu entry just to have it change right under my cursor making me click on something else because the widget was actually loading data asynchronously, without giving me any notice to the fact at all, is frankly ridiculous.

It's the right thing to do to load resources asynchronously in parallel, but you shouldn't load the interface piecemeal. Even on web browsers.

I'd much rather wait for an interface to be reliable than have it interactive immediately but having to make a guess about its state.