Improve overlay keydown handler (TAB-navigation)

Nice! - this mostly seems to work well, although tabbing to the close button doesn't seem to work correctly for me... can you reproduce that? Overall though it seems like this will help accessibility a lot.

The code looks good - I think the code comment might need a little more explanation, though. The "focusable which is right before the next focusable" wording seems confusing to me... ? I think what we're actually saying here is something more like whenever the focus is inside the document but not inside the overlay (or an overlay displaced region) we move the focus along until it is.

+    var allowedFocusableParent = '.overlay-displace-top, overlay-displace-bottom, #overlay-container';

Are you missing a "." in front of overlay-displace-bottom?

It's also a little sad that we have so many different ways for these "semi-overlay" regions to declare themselves. The overlay-displace stuff is already one, which this patch is reusing in a different way, then there's also the 'overlay_supplemental_regions' key, etc. It would be great to unify them into one thing on the PHP side and then have the overlay module take care of putting in the classes to make sure that all this other stuff works correctly. However, that's a preexisting problem and probably out of scope for this patch.

FWIW, it would appear to me that using focus handling to keep someone in the overlay when not using aria won't work.

With ARIA:
Those screen-readers that support 'application' mode will pass all keystrokes to the browser, and they can be handled.

Without ARIA:
Many screen-readers use a virtual document and don't pass all keystrokes to the browser, as they are used for interacting with and navigating the virtual
document.

But I *can* tab through the overlay without this patch :s

If you go to the skit to main content area left upper corner you can reach the overlay with or without the patch.
With the patch I still can't tab directly to the toolbar... (cleared cache).

After some cache clearing sessions...
It works :)

==> rtbc

Committed to CVS HEAD. Thanks.

The patch was never committed.

Committed to CVS HEAD. Thanks.