Plugin and theme editor focus trap

[afercia]

Reported by Michelle DeYoung during last accessibility testers group testing session:

"I'm not sure what is causing the tab order to stop at the textarea element and not allow the user to tab to the next form element in IE and Chrome. I tried using 'ESC' then 'Tab' to see if that would allow me to exit the textarea, but that only worked in FF."

Reproduced in Chrome and IE 8:

• go in plugin-editor.php
• click or tab inside the textarea
• type something
• click or tab outside the textarea (press Escape and then Tab)
• everything works OK

Refresh and repeat the previous steps and once inside the textarea:

• insert a tab character (press Tab)
• click or tab outside the textarea (press Escape and then Tab)
• the textarea is permanently focused

Keyboard users will be trapped, there's no way to move outside of the textarea using only the keyboard.

In the attached patch, I'd propose to simple remove the "lastkey" related stuff, I guess it was meant for Opera 12 and maybe no more necessary unless I'm missing something. Added also a little fix for Opera 27.

[Cheffheid]

a11y tests confirm that the patch fixes the trap. There are some minor quirks with some browser/screen reader combos where the ESC key needs to be pressed twice before the user can continue.

This might need to be reflected in the description.

<p id="newcontent-description">In the editing area the Tab key enters a tab character. To move below this area by pressing Tab, press the Esc key followed by the Tab key. In some cases the Esc key will need to be pressed twice before the Tab key will allow you to continue.</p>

[DrewAPicture]

Sounds like we may just need a new patch that address the help tab string change suggested in comment:7.

In the editing area the Tab key enters a tab character. To move below this area by pressing Tab, press the Esc key followed by the Tab key

[afercia]

Refreshed patch with the help tab string changes. FWIW, the need to press Escape twice may depend on the screen reader being in "forms mode" or "browse mode".

[afercia]

Help text changes.

[DrewAPicture]

31415.2.patch​ looks good, thanks @afercia.

[DrewAPicture]

@azaozz: Would you mind reviewing the JS changes here, see if there are any obvious gotchas?

[azaozz]

In 31908:

Fix focusing in the plugin and theme editors. Update the help.
Props afercia, Cheffheid. Fixes #31415.