ENG-9676 fix: route memo RestProp CSS props into css instead of dropping them

[FarhanAliRaza]

A rx.RestProp only forwarded kwargs that matched a declared field of the target component; any other forwarded prop (e.g. font_weight) was emitted as a raw plain prop the target silently ignored. Track each RestProp target's field names during body evaluation so the call site can fold non-field props into emotion css, matching how a normal component handles unknown kwargs.

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  fixes @rx.memo with rx.RestProp doesn't handle CSS props correctly #6599

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing FarhanAliRaza:style-fix (b89f189) with main (3653b8e)}

[greptile-apps]

Greptile Summary

Fixes a silent data-loss bug where rx.RestProp only forwarded kwargs that matched a declared field of the target component; any other kwarg (e.g. font_weight) was emitted as a raw camelCased plain prop that the target silently ignored. The fix tracks each RestProp target's field names during body evaluation and routes non-field kwargs through convert_dict_to_style_and_format_emotion into an emotion css prop, matching how a direct component call handles unknown kwargs.

• _rest_target_fields (a set[str]) is created before body evaluation and passed by reference through the lazy/eager lambda so _lift_rest_props can populate it as a side effect; rest_target_field_names() forces that evaluation before take_rest reads the set.
• take_rest is extended with a second parameter: fields in rest_target_fields become camelCased plain props as before; everything else is collected and wrapped in a single css key via emotion formatting.
• Unit and integration tests are updated/added to cover the CSS-forwarding path and the guard that real target fields remain plain props.

Confidence Score: 4/5

Safe to merge; the fix addresses a real silent-drop bug with a well-scoped, well-tested implementation and no regressions on common paths.

The core routing logic is correct: the shared-set mutation pattern for the lazy body, the field-vs-CSS classification, and the emotion formatting path all behave as intended. Two edge cases lack test coverage — convert_dict_to_style_and_format_emotion returning None when all style values normalise away, and memos with multiple RestProp targets whose field unions could misclassify a prop. Neither is likely to surface in typical usage, but both are worth hardening.

The take_rest method in memo.py around the convert_dict_to_style_and_format_emotion call (line ~1003) and the _lift_rest_props accumulator logic (line ~1095) deserve a second look for the None-return and multi-target edge cases.

Important Files Changed

Filename	Overview
packages/reflex-base/src/reflex_base/components/memo.py	Core fix: tracks rest-target component field names during body evaluation, then routes non-field kwargs to emotion css instead of silently forwarding them as plain props. Logic is sound and all paths (eager and lazy body) are handled.
tests/units/components/test_memo.py	Adds two new unit tests and updates an existing assertion to match the new routing behavior. Covers both the CSS-forwarding path and the "real target field stays as plain prop" guard case.
tests/integration/test_memo.py	Integration test now passes font_weight="bold" through rx.RestProp and asserts the computed CSS value is "700"; small, focused addition that covers the real-browser path.

Comments Outside Diff (1)

1. packages/reflex-base/src/reflex_base/components/memo.py, line 1095-1100 (link)

   Multiple RestProp targets: field union may misclassify props

   When a memo body spreads rest onto two different components (e.g. rx.box(rest) and rx.text(rest)), rest_target_fields becomes the union of both components' fields. A kwarg that is a field of Box but not Text would then be forwarded as a plain prop even when the usage context targets Text, silently passing an unrecognised prop instead of routing it to css. The docs call this out in the accumulator comment, but the behaviour can surprise authors of multi-target memos. If this is intentional, a short note in the rest_target_field_names docstring would clarify the trade-off.

   Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

_{Reviews (1): Last reviewed commit: "fix: route memo RestProp CSS props into ..." | Re-trigger Greptile}

[greptile-apps]

convert_dict_to_style_and_format_emotion can return None

format_as_emotion returns None when its internal emotion_style dict ends up empty after processing (e.g. all entries are filtered or a Var-only style resolves to nothing). Passing None to LiteralVar.create would produce a css={null} prop in the JSX output, which silently drops all emotion styles at runtime. The if css: guard only checks the pre-conversion dict, not the output. A None check on the result would make this safe.