jQuery UI Resizable broken in admin since 3.9

[cfinke]

As of WordPress 3.9, using the included jQuery UI Resizable plugin (jquery-ui-resizable) to make an element resizable in the admin does not work.

By "does not work," I mean that if you use the plugin to make an element resizable like so:

$( element ).resizable();

the element appears and acts no differently than if you had done nothing, but no errors or warnings are shown in the Console.

What should happen is that, when hovering over the borders of the element, the cursor should change to indicate resizability, and clicking and dragging the border should resize the element in that border's direction.

For an easy proof of concept, I've attached a plugin that adds a red rectangle to the top of every admin page. It should be resizable, and in WordPress version 3.8 (up through 3.8.5), it is (at least on post-new.php -- it's not resizable on edit.php, which may or may not be related to this bug). In versions 3.9 through 4.1.2, it is not.

Tested in the current stable versions of Firefox and Chrome on Mac OS X 10.10.3.

[cfinke]

Proof of concept plugin.

[helen]

We don't include any jQuery UI styling for things we're not using - possibly we were using it in the past and dropped it. I would not consider much, if any, of our current jQuery UI styling generically usable, so plugins should always be including their own. If I style .ui-resizable-handle to have a size and background so I can actually "grab" it, it works. Going to call this a dupe of #18909.