[@mantine/core] next try fixing transition issues

[cyantree]

(Hopefully) fixes #3126 #5193 #5849.

As I found out earlier it's probably related to the automatic batching in React. Therefore this fix uses flushSync() to enforce the state update at the right moment.

I hope that this will fix the issue.

[rilrom]

Nice work @cyantree! I can no longer reproduce the issue.

I'm curious why the changes to modal tests were necessary?

[cyantree]

I think that's because at first the component is unmounted and therefore the state is exited. This leads to the children not being rendered at all. This in effects leads to the test failing.
https://github.com/mantinedev/mantine/blob/master/packages/%40mantine/core/src/components/Transition/Transition.tsx#L71

This is because with my changes the state transition from exited to pre-entering doesn't happen in the same frame anymore (this is done because ReactDOM.flushSync() can't be used within a running render pass).

So setting transitionDuration: 0 disables the transition and renders the children directly. IMHO that's an acceptable behaviour.

However I'm happy for any suggestions as this (at least for me) is a finicky bug.

[macjuul]

I can also confirm it solves the problem on my end, thanks so much!

[rtivital]

Thanks everyone for validation! @cyantree, thanks for the fix!