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supabase/apps/docs
Jonathan Fulton 25f9f7c5e3 docs(realtime): add note about sync event behavior in Presence (#42338)
## What kind of change does this PR introduce?

Documentation improvement for Realtime Presence.

## What is the current behavior?

The Presence documentation doesn't explain that during a `sync` event,
clients may receive `join` and `leave` events simultaneously even when
no users are actually joining or leaving. This can be confusing for
developers new to Presence.

## What is the new behavior?

Added an explanatory note clarifying that:
- During `sync`, you may receive `join` and `leave` events at the same
time
- This is normal and expected
- It reflects state reconciliation with the server, not real user
movement

This helps developers understand this behavior upfront rather than
having to discover it through debugging.

## Additional context

As mentioned in the issue, this behavior is discussed in [this community
discussion](https://github.com/orgs/supabase/discussions/26748) where
the solution had to be discovered by users.

Fixes #41175

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Clarified behavior during sync events in the Realtime Presence guide.
Added explanation that simultaneous join and leave events may occur due
to state reconciliation rather than actual user movement changes.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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