Provide examples of types of menus

[scottaohara]

Closes #1084

Updated menu content to attempt to help differentiate menu and menubar, per the request of the original issue.

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Preview | Diff

[mcking65]

I think this should address more of the most misunderstood aspects of appropriate menu usage and reference the APG. I provided a specific suggestion.

[mcking65]

I think we can further simplify and clarify this language. I believe it is also important to specify that activating menuitems close menus, to enumerate how menus are triggered, and provide a link to APG.

Suggested change

	<p>A menu is a popup list of items that represent a set of actions or functions a user can invoke. The menu and list of <code>menuitems</code> are almost always directly related to the item that the user interacted with to reveal the menu. The <code>menu</code> <a>role</a> is appropriate when a list of menu items is presented in a manner similar to a menu on a desktop application. For instance, a menu could be used to represent a context menu for its invoking element, or it would be used to render sub-menus for items of a <rref>menubar</rref> or another <code>menu</code> popup.</p>
	<p>A menu is a popup containing a set of <code>menuitem</code> elements that can be invoked to perform an action or function. The actions are nearly always directly related to the element that the user activated to popup the menu. The <code>menu</code> <a>role</a> is appropriate when a popup is presented and behaves in a manner similar to a menu on a desktop application. activating a <code>menuitem</code> both performs the action and causes the popup to close.</p>
	<p>Menu popups can be triggered by:</p>
	<ul>
	<li>Activating a <code>menuitem</code> that has <pref>aria-haspopup</pref> set to <code>true</code> in a parent <code>menu</code> or <rref>menubar</rref>.
	<li>Activating a <rref>button</rref> that has <pref>aria-haspopup</pref> set to <code>true</code>.
	<li>Executing a command that triggers a context menu for an element.</li>
	</ul>
	<p>See the <a href="https://www.w3.org/WAI/ARIA/apg/patterns/menubar/">WAI-ARIA Authoring Practices</a> for more details about implementing menus.</p>

[scottaohara]

Is it odd to link to the APG page that only has menubar examples, from the menu definition?

not part of my previous PR, but this suggestion made me realize do we need to change the instance of <code>menuitem</code> to just say "menu item" - so as to not imply that only menuitem elements are allowed, and not also menuitemradio and menuitemcheckbox?

For the activation of menuitems, does that belong in the menuitem section? In other words, taking menuitemcheckbox into consideration, is it always true that activating one of those should cause a menu popup to close?

Pending those above questions, I'm otherwise good with these suggestions. I just don't know if they all belong in just this menu update?

[scottaohara]

@mcking65 re-ping on my questions concerning your suggested updates, so we can move this along

[MrHBS]

I would love to see this PR move forward.