Menu Customizer: reorder buttons tabindex not set for just added items

[afercia]

Just noticed when you add a new item to a menu, the just-added item reorder buttons don't get any tabindex attribute. This results in all 4 reorder buttons being focusable when using a keyboard. See screenshot below.
When adding more than one item, seems just the last one doesn't get the tabindex attributes.

[ocean90]

It looks like a new menu item doesn't get added to menuSection.container because console.log( menuSection.container.find( '.menu-item' ).length ); in reflowMenuItems returns the number of the initial added items.

[celloexpressions]

Anyone able to patch this?

[valendesigns]

The issue here seems to be a race condition. The menu item is not yet added to the UI when the tabIndex code is executed. By adding a short delay in 32943.diff​ I was able to get things to work correctly.

[westonruter]

@valendesigns In 32943.2.diff​ I introduce a new Customizer event pane-contents-reflowed which gets triggered when wp.customize.reflowPaneContents() finishes. The menu section then listens for this event and updates the tabIndex attributes at this time. This should be more robust. I'm noticing, however, that while the tabIndex attributes are getting set to -1, the keyboard focus sometimes isn't getting set to an adjacent element that *is* focusable. I suppose this would be a separate issue.

[valendesigns]

@westonruter Updated patch looks good!

[westonruter]

In 33283:

Customizer: Defer updating nav menu item tabIndex attributes until pane is reflowed.

Introduces pane-contents-reflowed Customizer event.

Props valendesigns, westonruter.
Fixes #32943.