Combobox

An enhancement of a textbox
Select phone model item specifics
Introduction
An enhanced textbox that allows free text input, selection from a predefined list of options, or a combination of both. Hence the name "combobox". It may also optionally have autocomplete behavour.
Updated: May 15th, 2021
Terminology
combobox: the pattern as a whole, comprised of the following distinct parts
textbox: stores and displays the form value
flyout: the overlay that contains a listbox
listbox: listbox containing options
option/suggestion: a suggestion inside of the listbox and/or directly after the textbox value
autocomplete: the autocomplete type (optional)
filter: the filtering criteria (optional)
Configuration
autoSelect: a combobox with autoSelect will automatically select and fill the textbox value when user cycles through listbox options. Otherwise, ENTER key is required to manually select an option. Typically autoSelect will be true for a combobox with autocomplete behaviour.
Working Example
Experience the pattern in action on our companion eBay MIND Patterns examples website.
Examine the required markup structure in our Bones GitHub project.
View a fully styled example in our eBay Skin CSS framework.
Best Practices
Each row in the list of options performs only a singular action: setting the value of the textbox. It is not possible, at the time of writing, to have additional actions per row, e.g. add, edit or delete.
The combobox listbox is not intended for storing or display kind of single-select or multi-select state. Again, the purpose of each option is to simply set the value of the textbox.
A combobox is an enhancement of textbox. Likewise, autocomplete is an enhancement of combobox. Suggestions provided will update dynamically based on user input. The suggested values may appear inline within the textbox, in a list, or both places at once.
Examples of autocomplete are the URL bar in browsers, and the main search field in search engines.
Interaction Design
Keyboard
When the combobox receives focus, the listbox should expand to show all options.
With listbox expanded, pressing DOWN-ARROW and UP-ARROW keys must navigate through the list of options. The keyboard focus will appear to be in two places at the same time - the textbox and the listbox. In actual fact, keyboard focus always stays on the textbox. The aria-activedescendant property controls the pseudo-focus inside of the listbox.
Pressing SPACEBAR must always enter a blank space in the textbox.
For a combobox with autoSelect, changing the highlighted option will automatically fill the textbox with that option.
For a combobox without autoSelect, changing the highlighted option must not automatically fill the textbox; ENTER key is required to manually select the option.
Pressing ENTER key while an option is highlighted must collapse the listbox. For a combobox with autoSelect the form will be submitted. For a combobox without autoSelect the form must not be submitted.
Pressing ESC key while an option is highlighted must collapse the listbox.
For autocomplete type "list":
The listbox remains but the options change based on the custom filtering criteria.
With focus in the empty combobox, type any letter. Any suggestions that match the filter will appear as options in the listbox flyout.
For autocomplete type "inline":
The listbox is removed. The entire combobox value will update as the user types, with the suggested portion highlighted as a selection range.
For autocomplete type "both":
This section is under development.
Screen Reader
The screen reader will announce the input as "text edit", "combobox" or words to those effect, depending on level of ARIA support.
The screen reader will announce "expanded" or "collapsed", depending on level of ARIA support.
The screen reader will announce any additional programmatic description, depending on level of ARIA support.
The screen reader will announce the current value of the textbox.
Mouse and Touch
When the combobox receives focus, via click or tap, the listbox should expand to show all options.
Clicking or tapping an option will fill the textbox with that value and collapse the listbox without triggering a form submit.
Developer Guide
Combobox is a good example of progressive enhancement. Until JavaScript is loaded or initalised, the textbox operates as a regular textbox. For example, a user can still enter and submit a value using the plain old textbox. The ability to choose a value from a list of pre-defined options is considered the enhancement that will be available with JavaScript.
Our combobox follows the ARIA 1.0 specification, as it has less issues than the ARIA 1.1 version. See Resolving ARIA 1.1 Combobox Issues for more information.
Textbox
We start with a label and textbox.
<span class="combobox" id="combobox-0">
  <label for="combobox-0-input">Game Console</label>
  <input id="combobox-0-input" name="console" type="text" placeholder="Playstation 4, Xbox One, etc."/>
  <!-- listbox options will go here -->
</span>
We have added our elements inside of a .combobox wrapper element. This wrapper acts as our module root and hook for CSS & JavaScript.
Remember: the textbox does not yet have a role of combobox, it is added later with JavaScript.
A listbox element will be appended to this wrapper. It is up to you whether you wish to render this server-side or client-side. There are pros and cons to both approaches, which we will discuss below.
Listbox
The listbox may render on the server or the client. It is wise to put the listbox in a hidden state if rendering on the server. To do so, use the hidden attribute.
<div class="combobox__overlay" hidden>
  <ul id="combobox_0-listbox" role="listbox">
    <li role="option" id="nid-0">Playstation 3</li>
    <li role="option" id="nid-1">Playstation 4</li>
    <li role="option" id="nid-2">Xbox 360</li>
    <li role="option" id="nid-3">Xbox One</li>
    <li role="option" id="nid-4">Wii</li>
    <li role="option" id="nid-5">Wii U</li>
  </ul>
</div>
Using JavaScript we now begin converting the textbox to a combobox, by adding role=combobox. We also create the properties and state that connect the combobox to the listbox:
<input id="combobox-0-input" name="console0" type="text" placeholder="Playstation 4, Xbox One, etc." role="combobox" aria-expanded="false" autocomplete="off" aria-owns="combobox_0-listbox">
The new attributes are role, aria-expanded, autocomplete and aria-owns.
Description
At the time of writing, screen readers don't do a good job of conveying the combobox role or how to interact with it. To solve this problem, we can add a description via aria-roledescription. 
<input aria-roledescription="Use up and down arrow keys to navigate combobox options" id="combobox-0-input" name="console0" type="text" placeholder="Playstation 4, Xbox One, etc." role="combobox" aria-expanded="false" autocomplete="off" aria-owns="combobox_0-listbox">
Now the screen reader announces the description whenever focus lands on the combobox.
If the number of options will change based on the value of the combobox (i.e. filtering occurs), the description should also make the user aware of this:
<input aria-roledescription="Use up and down arrow keys to navigate combobox options. Combobox options may change based on input." id="combobox-0-input" name="console0" type="text" placeholder="Playstation 4, Xbox One, etc." role="combobox" aria-expanded="false" autocomplete="off" aria-owns="combobox_0-listbox"/>
Keyboard and Screen Reader Navigation
Our elements are now in place, but how does a keyboard user navigate to the options? We cannot use TAB key because focus must stay on the combobox (so that user can type and enter their own value). As with most complex widgets, the answer lies in the arrow keys. Up and down arrow keys are the way to select our combobox options.
If focus must remain on the combobox, how then do we also have focus on the listbox options? The answer is that we don't. Focus always remains on the combobox and instead we have a kind of pseudo-focus on the options.
How does the screen reader know where this pseudo-focus is?
Active Descendant
We call the option with pseudo-focus the "active descendant". And guess what, there is an ARIA attribute for this called aria-activedescendant. This attribute is placed on the combobox element. The attribute value is the ID of the currently active (pseudo-focussed) option. This allows assistive technology such as a screen reader to programmatically determine
To make all of this easier, we recommend using a plugin such as makeup-active-descendant. After your HTML structure is in place, simply initialise the plugin on the widget and up/down arrow keys will update the necessary states. Use CSS to style the active descendant in any way you like.
ARIA Reference
This section gives an overview of ARIA usage, within the context of this pattern.
role=combobox
This attribute changes the role of the text input from textbox to combobox. We recommend applying this attribute on the client-side with JavaScript.
aria-roledescription
Gives additional instructions on how to operate the combobox options.
role=listbox
The list of suggestions has a role of listbox.
role=option
Each listbox item has a role of option.
aria-owns
This property creates a programmatic hierarchy in the accessibility tree for the combobox and the listbox.
aria-expanded
Conveys the expanded state of the combobox.
aria-label
Provides the expand/collapse button with an accessible label, in the case where it has no visible text (i.e. an icon button).
Further Reading
​https://github.com/w3c/aria/wiki/Resolving-ARIA-1.1-Combobox-Issues​
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