focus: autofocus should work with shadow hosts with delegatesFocus an…

[tkent-google]

…d area elements

Fixes #5027

• At least two implementers are interested (and none opposed):
  • Chrome: Yes
  • Firefox: Yes
• Tests are written and can be reviewed and commented upon at:
  • autofocus: Add tests for delegatesFocus and <area>. web-platform-tests/wpt#20083
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1024056
  • Firefox: Existing https://bugzilla.mozilla.org/show_bug.cgi?id=1574435 is enough
  • Safari: Existing https://bugs.webkit.org/show_bug.cgi?id=203139 is enough

(See WHATWG Working Mode: Changes for more details.)

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/interaction.html ( diff )

[domenic]

This looks great to me. (I pushed some editorial tweaks; please take a look to make sure I didn't mess anything up.)

For anyone else watching, the diff is a bit hard to read, but the essence of it is simple: extract out step 1 of the focusing steps, which translates focus targets into focusable areas, into its own algorithm (named "get the focusable area"). Then, call that from the autofocus algorithm, so that autofocus works on "exotic" cases like <area> and delegatesFocus which need that extra translation step.

@rniwa @smaug---- can you confirm that you're up for making autofocus work with <area> and delegatesFocus in WebKit and Gecko? In my opinion this is a pretty simple bugfix and I'd like to merge ASAP.

[tkent-google]

This looks great to me. (I pushed some editorial tweaks; please take a look to make sure I didn't mess anything up.)

The edit looks great. Thank you!

[smaug----]

(huh, github's reviewing tool really can't deal with this change well)