Implement new focusing steps for modal <dialog>.

Implement new focusing steps for modal <dialog>.

When a modal <dialog> is opened, then
1) The first autofocusable element in the dialog gets focus, or else
2) The first focusable element in the dialog gets focus, or else
3) The dialog itself gets focus, if it's focusable, or else
4) Focus is cleared (effectively, document.body gets focus)

I chose to do (4) since the the previous implementation
relied on CheckFocusedElementTask to clear focus, which
isn't great because "dialog.showModal(); document.activeElement"
was a race condition. One problem with (4) is it will
remove focus from a non-inert ancestor, but the spec also recently
changed to make ancestors inert so that's the direction to take.
We can't make that change immediately though since:
a) it would make document.body inert, which violates
assumptions of document.body getting default focus when
nothing else can have focus, and
b) it would prune the modal <dialog> out of the accessibility
tree

As per the recent spec change:
http://html5.org/r/8338

BUG=298078, 324401
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=163796

[falken, 2013-12-12 08:04:02]

Dominic: could you please review? This obsoletes the focus event forcing I was
working on in https://codereview.chromium.org/101193002/

Elliott: FYI

[falken, 2013-12-12 08:31:10]

+dominicc

[dmazzoni, 2013-12-12 16:04:49]

New description sounds good, but I don't see the new code - did the upload
fail?

On Thu, Dec 12, 2013 at 12:31 AM, <falken@chromium.org> wrote:

> +dominicc
>
> https://codereview.chromium.org/112553002/
>

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[dmazzoni, 2013-12-12 16:07:02]

Oh, nevermind, I was looking at the wrong change.


On Thu, Dec 12, 2013 at 8:04 AM, Dominic Mazzoni <dmazzoni@chromium.org>wrote:

> New description sounds good, but I don't see the new code - did the upload
> fail?
>
> On Thu, Dec 12, 2013 at 12:31 AM, <falken@chromium.org> wrote:
>
>> +dominicc
>>
>> https://codereview.chromium.org/112553002/
>>
>
>

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[dmazzoni, 2013-12-12 16:12:54]

lgtm

Great!

It sounds like we may still need an accessibility fix if the dialog has no
focusable children and isn't itslef focusable?

[commit-bot: I haz the power, 2013-12-12 16:13:00]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/falken@chromium.org/112553002/50001

[commit-bot: I haz the power, 2013-12-12 17:08:56]

Change committed as 163796

[falken, 2013-12-13 01:45:11]

On 2013/12/12 16:12:54, Dominic Mazzoni wrote:
> lgtm
> 
> Great!
> 
> It sounds like we may still need an accessibility fix if the dialog has no
> focusable children and isn't itslef focusable?

This case actually turns out pretty good for NVDA-- it reads the document title
followed by "dialog". I think what fixed it is now we send the focus event after
throwing away the cache.

I'd still like to get rid of reading the document title each time dialog is
opened/closed, but we're improving.