Visual Feedback for Nav Menu UI

[sterlo]

Currently if you go to:
/wp-admin/nav-menus.php

Then click the plus sign to add a Menu and then without putting a menu name in, click create menu, it gives you a visual indicator that you've missed a critical bit of data.

But, if you try to add a Custom Link to an existing menu: If one forgets to add a URL and just adds a label then tries to save it, there is not 'tactile feedback'. It just sits there not doing anything.

It should probably be consistent with how the rest of the UI behaves.

[nacin]

Agreed.

[jane]

Agreed, though it needs visual feedback, not tactile.

[pragunbhutani]

I've added an addClass('form-invalid') to the event handler in nav-menu.js
Also updated class definition for .form-invalid to include .customLinkDiv

[helen]

In 32690:

Nav menus: Provide visual feedback when attempting to add an empty custom link.

props pragunbhutani for the initial patch.
fixes #18517.