perf(handler): stop native attributes leaking into component state

[Mohamed Mansour (mohamedmansour)]

Problem

process_attribute accumulated every non-skipped attribute into context.component_attrs, but only component opening tags carry attr_start. Attributes on native elements were collected too, and the next component fragment adopted them as local variables.

<div title="{{nativeTitle}}"></div>
<my-comp></my-comp>   <!-- {{title}} resolved to the div's value, not state -->

<my-comp> declares no attributes at all, yet rendered native instead of the global title. Native element state silently leaks into the next component's scope.

Fix

A new collecting_component_attrs flag on WebUIProcessContext scopes the accumulation window:

• attr_start opens the window (only the parser sets it, and only on component elements).
• Entering a component closes it, so trailing native attributes cannot leak into a sibling.
• The streaming VM clears it on component enter and exit, so a leak cannot cross a component or a chunk boundary.

Every component_attrs.insert is now gated on the flag. attr_skip handling is unchanged.

Allocation effect

attr_start previously did context.component_attrs = HashMap::new(), discarding the capacity-preserving map that process_component/begin_component had just pulled from the scope pool. The pooling was defeated on the very next attribute. Clearing instead of replacing restores it.

A deterministic counting-allocator harness over 500 component instances, measuring only the render call:

	allocations	bytes
main	2003	129,815
this PR	1505	8,303
delta	-498 (-24.9%)	-121,512 (-93.6%)

Exactly one allocation per component instance is avoided.

No timing claim is made. Wall-clock spread on this machine exceeds the effect size: running pristine main against its own saved criterion baseline reproduces the same 3-19% "regressions" on handler_state_depth, so those readings are machine noise, not signal. The deterministic allocation counts above are the claim, and there is no measured regression attributable to this change.

Tests

Two regression tests, both verified failing on pristine main and passing here:

• native_dynamic_attribute_does_not_leak_into_next_component (direct render). Main produced <my-comp>native</my-comp>, expected global.
• native_attribute_does_not_leak_into_next_streamed_component (streaming VM path). Main produced <span>native</span> inside the boundary chunk.

Output is byte-identical to main everywhere except the intentionally fixed leak case, and the full workspace suite passes unchanged.

Gate

cargo xtask check: license-headers, fmt, clippy, proto (drift check), deny, test, build, build (wasm), build (examples), bench (validate) all pass.

The docs phase fails with PROJ-C013: Adapter module graph is incomplete or inconsistent. This is pre-existing and unrelated. The identical failure was reproduced on pristine main (2efb60e) before any change here, and is deliberately not addressed in this PR.

Scope

Touches only the handler attribute-collection gate plus its DESIGN.md contract note. No manifest, lockfile, benchmark, or example changes.

[Copilot]

Pull request overview

Fixes a correctness bug in webui-handler where native HTML attributes were being accumulated into component_attrs and could leak into the next component’s local-variable scope, and also restores intended HashMap pooling behavior for fewer allocations during render.

Changes:

• Add collecting_component_attrs to explicitly scope when component_attrs may be populated (opened by attr_start, closed on component enter/exit and on new streaming contexts).
• Avoid defeating the scope-map pool by clearing (not replacing) component_attrs on attr_start.
• Add regression tests covering both direct render and streaming VM paths, and document the contract in DESIGN.md.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
DESIGN.md	Documents the attr_start/attr_skip “component attribute collection window” contract and clarifies that native attributes cannot become later component locals.
crates/webui-handler/tests/streaming_v2.rs	Adds a streaming regression test to ensure native attributes don’t affect subsequent component state resolution.
crates/webui-handler/src/streaming/vm.rs	Clears collecting_component_attrs on component enter/exit to prevent state leakage across component boundaries in the VM path.
crates/webui-handler/src/streaming/session.rs	Initializes collecting_component_attrs to false when constructing the per-step streaming render context.
crates/webui-handler/src/streaming/inventory.rs	Updates test context construction to include the new collecting_component_attrs field.
crates/webui-handler/src/lib.rs	Introduces and gates attribute accumulation behind collecting_component_attrs, clears pooled maps on attr_start, and adds a direct-render regression test.

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