Twenty Thirteen: Make the menu keyboard accessible

[Frank Klein]

Currently it is not possible to access the drop down menus in Twenty Thirteen using the keyboard. The attached patch fixes this.

It also changes the way that the display of the drop down menus is handled. The problem with using display: none is that the browser doesn't take these elements into account when determining tab order.

If you use another CSS technique for handling this (is Twenty Fourteen does), the tab order will be correct, i.e. it will follow the structure of the lists of which the navigation is composed. You can test this by using Tab and Shift+Tab to navigate forward and backward through the menu items.

[Frank Klein]

Updated patch that fixes issues with the responsive small menu styles.

[sabreuse]

related: #27011

[rhyswynne]

Applied this patch to WordPress Trunk (3.9-alpha-27234-src). Tested it on a Windows 7 with Google Chrome (33.0.1750.117), Internet Explorer (11.0.9600.16518) and Firefox (27.0.1

Works fine and as expected.

[joedolson]

This patch will cause problems in RTL languages -- setting the off-left position will generate a scrollbar under RTL reading order. I've attached an alternate patch I created for #24839 that uses the clip technique to hide the menus.

[joedolson]

Alternate patch for #27039; also attached to #24839

[lancewillett]

Duplicate of #24839.

[lancewillett]

Fixed in r27607.