fix: route frontend events from middleware preprocess updates to the client

[FarhanAliRaza]

A blocking middleware (e.g. an auth gate) can return a StateUpdate whose events are frontend-only specs like rx.toast or rx.redirect. These have no registered backend handler, so enqueueing them on the backend queue raised KeyError. Extract the frontend/backend routing from chain_updates into _route_events and reuse it when applying preprocess updates.

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• Bug fix (non-breaking change which fixes an issue)

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[greptile-apps]

Greptile Summary

This PR fixes a bug where frontend-only events (e.g. rx.toast, rx.redirect) returned from a middleware's preprocess were passed directly to the backend event queue, causing a KeyError because those specs have no registered backend handler. The fix extracts the existing frontend/backend routing logic from chain_updates into a shared _route_events helper, then applies it to the middleware-preprocess path.

• _route_events splits events by whether their name starts with _, emitting frontend events to the client and enqueueing backend events, mirroring the routing already done inside chain_updates.
• The middleware preprocess block in _execute_event now calls _route_events instead of the bare ctx.enqueue(*update.events).
• A new integration-style regression test verifies _call_function and _redirect events reach emitted_events (the client) rather than the backend queue.

Confidence Score: 5/5

Safe to merge — the change is a minimal, targeted extraction of existing routing logic into a shared helper, and the new middleware-preprocess path is covered by a regression test.

The fix is a straightforward refactor: the same name.startswith("_") split that already worked correctly in chain_updates is now reused for the middleware-preprocess path. The only runtime behaviour change in chain_updates itself is that ctx.enqueue() is no longer called with zero arguments when all yielded events are frontend-only — a clean improvement with no observable downside. The regression test exercises the exact failure scenario described in the PR.

No files require special attention.

Important Files Changed

Filename	Overview
packages/reflex-base/src/reflex_base/event/processor/base_state_processor.py	Introduces _route_events helper and applies it in both chain_updates and the middleware-preprocess branch; logic is correct and well-guarded.
tests/units/reflex_base/event/processor/test_base_state_processor.py	Extracts wired_app fixture for reuse and adds a targeted regression test for the middleware-preprocess frontend-event routing bug.
packages/reflex-base/news/6644.bugfix.md	Changelog entry accurately describes the bug and its fix.

_{Reviews (2): Last reviewed commit: "refactor: single-pass event partition an..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing FarhanAliRaza:fix-routing-events (95fe2fb) with main (2f95065)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩