Facebook's address auto-completion in the compose message form is one of the best examples of auto-completion behavior in a form. The interaction very closely mimics what you would expect to happen in the To field of email applications like Apple Mail and Outlook.
1. Cursor is in the To: field. Contextual help displayed below input.
2. Typing characters in the input invokes person/recipient auto-completion suggestions.
3. Selecting a person from auto-completion suggestions, entering a comma, or clicking enter displays the selected recipient in a block. The recipient's name cannot be edited, but it may be removed by using the backspace key or clicking the "x" next to the name.
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