Allow opting out of startTransition

[KurtGokhan]

Original issue here #12552, and paraphrasing my comment directly:

I brought myself up to date with the concurrency in React. I already had some information about them but I wasn't aware of the whole implications until I used them in a real application.

startTransition is not a silver bullet and it shouldn't be used blindly. There are cases where it is perfectly valid to disable it. Things that happened instantly before don't happen instantly anymore. It is a huge tradeoff in terms of complexity when the old way used to work perfectly for some use cases (most, considering we didn't have this feature before).

It is possible to make your entire app transition-aware. But this is too impractical, and often unnecessary. From my experience, transition-aware hooks/components are not easily composable. Also there are some bugs and unexpected behaviors that make it harder (examples facebook/react#31819, facebook/react#25814, and all caveats here).

Transitions are a feature of React, but so is useDeferredValue and useSyncExternalStore. I think it's not good to fixate on one solution and ignore others. You don't have to sacrifice old features to add new features.

I could keep using v6. Or I could stop overthinking and ignore sub-second state tearing issues and tiny lags like most of the slop out there. But I am asking you to reconsider, for the greater good. I am sure more people will (or worse, won't) notice these issues as they keep migrating to v7.

I am aware I made too much noise, but TLDR:

There should be a way to opt-out of startTransition.

After a few unsuccessful attempts to migrate to v7, my thoughts are still the same.

This issue can be solved in one of the two ways:

1. Allow passing an option to router context, navigation components and hooks to opt-out of startTransition.
2. Don't use startTransition, but internally use useDeferredValue to get the router state. Expose another hook like useLocationSync that can get the non-deferred router state so that users can opt-out and build their custom hooks upon that. This is more effort and I don't know if it would work well.

I think startTransition is incompatible with most client-side applications, and it will be too late when people start noticing this. Please don't let this be another nail in the coffin for client-side React.