When MediaViewer starts loading, it displays a black overlay. In rare situations, loading might take long; bugs might even cause it to hang. Users should see a progress bar that clarifies that the application is loading.
There is a progress bar for image loading (once the interface has been initialized), but not one for MediaViewer itself loading. There is no way to measure the progress of that, so it would have to be an indeterminate progress bar.
See also: T86287