[BUG] Loading a plugin from plugins directory flickers

[lizozom]

To reproduce:

1. Generate a new plugin using the plugin generator
2. Place a breakpoint in plugins/moo_123/public/application.tsx, before the render.
3. Start kibana and navigate to the plugin.

Result:

This screen shows:

Without the breakpoint, this screen flickers and disappears once the plugin renders.

[elasticmachine]

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

[pgayvallet]

This is very likely to be related to #54656. Sounds like the router is mounting the app slightly before the state observer emit? Not sure how or why though.

Also I'm not sure to see how there could be a difference between internal and 3rd parties plugins behavior.

[lizozom]

@pgayvallet I haven't tested whether this happens with a new non 3rd party plugin!

[pgayvallet]

closing in favor of #56409