fix: set and flushSync state that should render in if when another state is set breaks reactivity temporarily

[raythurnvoid]

In this PR i'm restoring the flushSync optimization introduced in #15855 that was replaced by #15895.

I reverted #15895 and adapted #15855 to the more recent changes of flushSync

In particular the problem seems to be that setting is_flushing to true before calling flush_tasks
prevents the reactivity to work normally when a state is update in a flushSync and that same value is used in an if block that should be rendered after setting another state right after the flushSync.

Here are relevant playgrounds:

Before the regression
https://svelte.dev/playground/a89aaf76146c4571a996d7cd02e39d2f?version=5.28.2

When the regression was introduced
https://svelte.dev/playground/a89aaf76146c4571a996d7cd02e39d2f?version=5.28.3

Actual version, still broken
https://svelte.dev/playground/a89aaf76146c4571a996d7cd02e39d2f?version=5.33.14

Unfortunately I failed to reproduce the issue in the jsdom env, however it can be clearly tested both with sv create and in the playground.

Do you plan to upgrade vitest and switch to browser mode in the future?

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Latest commit: 9c0a31f

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Playground

pnpm add https://pkg.pr.new/svelte@16077

[raythurnvoid]

This fixes #16076

[paoloricciuti]

@raythurnvoid sorry I missed that you had this already open... although the other one is a better fix...thanks anyway 🧡

[raythurnvoid]

@raythurnvoid sorry I missed that you had this already open... although the other one is a better fix...thanks anyway 🧡

Yeah your was definitely better no worries, I miss a lot of context when touching the reactive system! I hope the analysis of the issue was of any help at least 😅