Fix: Radio focus handler bound to onChange instead of onFocus

[TranscenderNing]

Summary

• Bug: MaterialRadio.prototype.init() line 230: boundFocusHandler_ was incorrectly bound to this.onChange_ instead of this.onFocus_
• Impact: Every focus event on a radio button (Tab key navigation) incorrectly triggers the change handler, causing O(n) DOM scan + potential visual state glitches
• Fix: Single word change: .onChange_ → .onFocus_

Root Cause Analysis

In src/radio/radio.js line 230:

// BEFORE (buggy):
this.boundFocusHandler_ = this.onChange_.bind(this);  // ← copy-paste error

// AFTER (fixed):
this.boundFocusHandler_ = this.onFocus_.bind(this);   // ← correct method

This is a clear copy-paste error from line 229, where boundChangeHandler_ is correctly bound to onChange_. The same pattern is correctly implemented in sibling components:

• checkbox.js line 251: this.boundFocusHandler_ = this.onFocus_.bind(this);
• switch.js line 252: this.boundFocusHandler_ = this.onFocus_.bind(this);
• radio.js line 230: this.boundFocusHandler_ = this.onChange_.bind(this); ❌

Reproduction Steps

1. Create a page with MDL Radio buttons
2. Use Tab key to navigate/focus a radio button
3. Before fix: Focus triggers onChange_ which does a full DOM query via document.getElementsByClassName() and calls updateClasses_() on every radio button on the page
4. After fix: Focus only triggers lightweight onFocus_ handler

Test Plan

• Verified onFocus_ method exists at line 97 of radio.js
• Confirmed binding pattern matches checkbox.js and switch.js
• Manual test: Tab-navigate through radio buttons - no unexpected DOM scanning
• Existing test suite should continue to pass (backward-compatible change)

Additional Context

This was found during a systematic code audit comparing event handler initialization across all MDL form components. All three primary form components (checkbox, radio, switch) share nearly identical initialization patterns, making this inconsistency easy to spot.

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