Platform Applications flashes "Application Not Found" UI when loading

[rudolf]

When loading a valid enabled application, we briefly render the following "Application Not Found" error page:

Seems to be caused by this line: https://github.com/elastic/kibana/blob/master/src/core/public/application/ui/app_container.tsx#L57

[elasticmachine]

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

[pgayvallet]

Related to #54656

[pgayvallet]

@jasonrhodes I tried to reproduce, by both accessing an initial NP app using a refresh, or navigating from either a NP or LP app to a NP one, and I can't see the flickering on my machine.

Is there a reproduction scenario for this?

[pgayvallet]

Confirmed. The first render is done before the status observable hook is initialized

kibana/src/core/public/application/ui/app_router.tsx

Line 46 in de7a22d

const appStatuses = useObservable(appStatuses$, new Map());

Causing an initial not found display until the first render is done:

kibana/src/core/public/application/ui/app_container.tsx

Lines 65 to 72 in 071db79

	unmountRef.current =
	(await mounter.mount({
	appBasePath: mounter.appBasePath,
	element: elementRef.current!,
	onAppLeave: handler => setAppLeaveHandler(appId, handler),
	})) || null;
	setAppNotFound(false);
	};

Moving the setAppNotFound(false); before the await seems to be enough to address the issue.

However, the time we await the actual mount handler to resolve will still display a blank content. This is way less impacting that displaying the error message, but we might want to display the loader while awaiting the mount. This is outside of the scope of this issue though, and minor.