Show loading indicator while application is mounting

[joshdover]

The ApplicationService in the Kibana Platform should show some indication that an application is loading while it's mount function is running. This is not a blocker for 8.0 but would improve the UX quite a bit since an application's mount function may take some time to resolve as it fetches any async import bundles.

The loading indicator should start showing when as soon as we start unmounting the previous application and should stop showing after the next application's mount function resolves. We can probably reuse the existing loading bar indicator that shows at the top of the page.

This should not require any API changes to the ApplicationService.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[mshustov]

It would be good to render a spinner only if loading takes more than a threshold (say ~250ms) to avoid flickering.

[pgayvallet]

mount: (context: AppMountContext, params: AppMountParameters) => AppUnmount | Promise<AppUnmount>

Is the mount function synchronous though? I would guess async bundle loading are... async, and therefor actually executed after the mount returns? Or should we consider that the returning promise resolving is the end of the loading stage?

[timroes]

Could you give some update on this issue please? Since now more and more application switch over to Kibana Platform this is becoming a real bad UX, since you basically are just clicking something and don't see any progress when having a slow(er) internet connection. I think we def should add a smart loading spinner soon, meaning a spinner that will have a short delay like 100-200ms before showing so for really fast switches you won't see it, and if it starts showing I would recommend keeping it showing for at least ~400-500ms (or fading it out there), even if we would load faster, since from my experience, that hiding it immediately once showing will create a really not nice flickering in situations.