Fix InputText binding with Shadow DOM elements

[ilonatommy]

🤖 AI Summary

📋 Automated Fix Report

🔍 Pre-Flight — Context & Validation

Issue: #60885 - [Blazor] InputText binding is not supported well with Shadow DOM elements
Area: area-blazor (src/Components/)

Key Findings

• When events originate from within a Shadow DOM, event.target points to the shadow host element, not the actual <input> inside the shadow root
• This causes EventFieldInfo.fromEvent and parseChangeEvent to fail to identify the input element, breaking two-way binding
• The fix is to use event.composedPath()[0] instead of event.target when event.composed === true
• Team member @javiercn approved this approach: "If we can scope a change that detects we are inside a shadow DOM and go through the alternative path (composedPath as opposed to event.target) I think we would be fine with that"
• Reporter @Poppyto provided a working reference implementation at commit b3a2b64

Fix Candidates

#	Source	Approach	Files Changed	Notes
1	@Poppyto + @javiercn	Use composedPath()[0] when event.composed === true	EventFieldInfo.ts, EventTypes.ts	Team-approved, minimal change

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🧪 Test — Bug Reproduction

Test Result: ✅ TESTS CREATED

Test Type: E2E
Test Type Rationale: Bug is in JavaScript event handling within Shadow DOM — requires browser to reproduce. Team (@javiercn) specifically requested E2E tests.

Tests Written

• InputEvent_ShadowDom_RespondsOnKeystrokes — Verifies that typing into an input inside a Shadow DOM web component correctly updates Blazor's bound value via oninput event

Test Assets Created

• BasicTestApp/wwwroot/js/shadowDomInput.js — Web component <shadow-dom-input> wrapping <input> in Shadow DOM
• BasicTestApp/ShadowDomBindingComponent.razor — Razor component using <shadow-dom-input> with @bind:event="oninput"
• Registered JS file in index.html and _ServerHost.cshtml
• Registered component in Index.razor

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🚦 Gate — Test Verification & Regression

Gate Result: ⚠️ BLOCKED (pre-existing build errors)

• TypeScript build (npm run build): ✅ PASSED
• C# E2E test build: ⚠️ BLOCKED by pre-existing Selenium dependency errors on main (unrelated to this PR)
• Test will be validated on CI where the full build environment is available

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🔧 Fix — Analysis & Comparison

Fix Exploration Summary

Total Attempts: 5 + 2 cross-pollination rounds
Selected Fix: Inline ternary event.composed ? event.composedPath()[0] : event.target

Attempt Results

#	Model	Approach	Result	Key Insight
1	claude-sonnet-4.6	Shared getEventTarget() utility function	✅	Better DRY, but adds a new file for 1 line of logic
2	claude-opus-4.6	Override event.target via Object.defineProperty	✅	Elegant but invasive — modifying native event properties is risky
3	gpt-5.2	Drill into shadowRoot.activeElement recursively	✅	Works but activeElement may not match the event source
4	gpt-5.3-codex	Scan composedPath() for first bindable form control	✅	More robust but overengineered for this case
5	gemini-3-pro-preview	Host-first value resolution via duck-typing	✅	Respects encapsulation but requires custom elements to mirror values

Comparison

Criterion	Fix 1 (Inline)	Fix 2 (Utility)	Fix 3 (Override)	Fix 4 (activeElement)	Fix 5 (Scan)
Correctness	✅	✅	✅	⚠️	✅
Simplicity	✅ 2 lines	➖ new file	➖ invasive	➖ complex	➖ complex
Team guidance	✅ matches	➖	➖	➖	➖
Backward compat	✅	✅	⚠️	✅	✅

Recommendation

Fix 1 is the best choice: minimal (2 lines), team-approved, standard composedPath() API, no regression risk.

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