Hello!
This is no insurmountable problem on my end, but I am implementing Babylon as a lightbox with no iframe. In other words, my canvas has its CSS position property set to "fixed". This breaks the placement of the loading screen.
Obviously I will patch it up on my end for now, but to make it work out of box, you could replace line 137 in https://github.com/BabylonJS/Babylon.js/blob/preview/src/Loading/babylon.loadingScreen.ts with something like:
var canvasPositioning = window.getComputedStyle(this._renderingCanvas).position;
this._loadingDiv.style.position = (canvasPositioning == "fixed") ? "fixed" : "absolute";
I'm pretty sure that would have it working out of box in my case.