Angular Elements syncs attributes to input properties, but not the reverse: assigning an input property never updated the corresponding attribute. This adds an opt-in reflectProperties option on NgElementConfig. When enabled, assigning a primitive value (string, number or boolean) to an input property writes it back to the matching attribute so the two stay in sync.

Non-primitive values are intentionally not reflected, since they cannot be meaningfully serialized to an attribute. A boolean true reflects as an empty attribute, while false, null and undefined remove it. A guard prevents the reflected attribute change from being fed back through attributeChangedCallback. The option defaults to false, preserving existing behavior.

Fixes #23557

PR Checklist

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Angular Elements keeps input properties in sync when an attribute changes
(setAttribute(...) updates the property), but not the other way around. Assigning
an input property never updates the corresponding attribute, so the property and
attribute drift out of sync.

Issue Number: #23557

What is the new behavior?

Adds an opt-in reflectProperties option on NgElementConfig. When enabled,
assigning a primitive value to an input property reflects it back to the matching
attribute so the two stay in sync:

• string / number → written to the attribute as a string
• true → empty attribute; false / null / undefined → attribute removed
• objects / arrays / functions are not reflected, avoiding meaningless
  [Object object] attributes (the concern raised earlier on the issue)

A guard prevents the reflected attribute change from being fed back through
attributeChangedCallback. Unit tests cover string, number, boolean, null/undefined,
non-primitive, aliased-attribute, and no-feedback-loop cases.

Does this PR introduce a breaking change?

• Yes
• No

The option defaults to false, so existing behavior is unchanged.

Other information

Open to shaping the API — the opt-in flag lives on NgElementConfig, but a per-input
opt-in would also work if the team prefers. Happy to adjust based on review.