Customize Menus: Add "Your theme already has a basic menu" view

[melchoyce]

When your theme creates a menu by default, we should display the following text:

Your theme already has a basic menu, to help readers find all your pages. If you’d like to choose what appears instead, create a new menu to replace the default.

Click the button to start.

[ Replace default menu ]

See screenshot.

[joyously]

How would the Customizer know whether the theme has used a fallback menu or not?
(Is that what is meant by "When your theme creates a menu by default", because themes in the repository aren't supposed to create menus?)

[westonruter]

This is something we'd be able to sniff for via in the preview. In particular, when wp_nav_menu() is called and falls back to listing pages, then a selective refresh partial will still be created. So in that case, the partial itself could send a message to the parent window to inform it that a given theme location has a fallback populating it.

[westonruter]

Correction: Fallback menus don't yet get selective refresh partials. There is a patch for this in #39665. In that patch there is a echo_preview_nav_menu_args_fallback_with_partial_refresh method that wraps the original fallback_cb and calls it. So inside of this function we'd be able to look to see if a theme_location param is present, and if so, then tally that location similar to how we ​tally sidebar calls and then ​export them to JS in the preview, and then it could be sent to the controls window via wp.customize.preview.send( 'nav-menu-locations-with-fallbacks', locations ) which then can be listened to in the controls pane via wp.customize.previewer.bind( 'nav-menu-locations-with-fallbacks', function() { … } ). This listener can then toggle whether or not the additional text should be displayed.

Nevertheless, this isn't going to land in in 4.9 due to the dependency here on #39665.