Specify popover=auto on ::picker(select)

[josepharhar]

This was accidentally missed on the original spec for base appearance.

Fixes #12271

• At least two implementers are interested (and none opposed):
  • Chromium
  • …
• Tests are written and can be reviewed and commented upon at:
  • All customizable select tests are based on this element being a popover
• Implementation bugs are filed:
  • Chromium: Already implemented
  • Gecko: Do we need a bug for this?
  • WebKit: Already implemented?
• Corresponding HTML AAM & ARIA in HTML issues & PRs: not needed
• MDN issue is filed: not needed
• The top of this comment includes a clear commit message to use.

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

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/rendering.html ( diff )

[keithamus]

LGTM.

[annevk]

Unfortunately this does not work as-is, since :popover-open should not match and this creates style collisions with [popover]. There might be more issues. I haven’t evaluated all entry points.