[FF] Shift + tab doesn't work in nested editable

[ma2ciek]

🐞 Is this a bug report or feature request? (choose one)

• Bug report

💻 Version of CKEditor

1.0.0-alpha2

📋 Steps to reproduce

Go to http://localhost:8125/ckeditor5-core/tests/manual/article.html.
Put the caret inside the image caption.
Press Shift+Tab

✅ Expected result

Caret should go to the first line of the editor.

❎ Actual result

Nothing happens.

📃 Other details that might be useful

Browser: Firefox 57 (Quantum).
Related: #676

[Reinmar]

It's not defined that tab/shift+tab should cycle through contentEditable elements. Whatever happens now is "ok" since it's native behaviour for now.

[ma2ciek]

Now, the focus is locked in the caption. It's not a native behavior, focus should go somewhere else after Shift+Tab

[f1ames]

I have noticed that Shift+Tab is locked only in the first image caption. If you have two images you can switch between captions with Tab and Shift+Tab. However, using Shift+Tab in the first image caption in the editor has no effect (checked on Firefox 59.0.2 (64-bit)).

[Reinmar]

Invalid. See #1709.