* Use `postMessage` transport for header image settings by default when video headers are supported, and thus `the_custom_header_markup()` will necessarily be used (and thus a selective refresh partial will be available).
* Ensure that `the_custom_header_markup()` always outputs a container element in the customizer preview even if the header is empty.
* Ensure that edit shortcut appearing for custom header does not get positioned off-screen.
Props bradyvercher, westonruter.
See #38639.
Fixes#38737.
Built from https://develop.svn.wordpress.org/trunk@39227
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This adds the ability for themes to add support for videos in custom headers
by passing `'video' => true` as an argument when adding theme support for
custom headers.
Custom video headers are managed through the “Header Visuals” (i.e. “Header Image”)
panel in the Customizer where you can select a video from the media library or set a
URL to an external video (YouTube for now) for use in custom headers.
This introduces several new functions:
`has_header_video()` – Check whether a header video is set or not.
`get_header_video_url()` – Retrieve header video URL for custom header.
`the_header_video_url()` – Display header video URL.
`get_header_video_settings()` – Retrieve header video settings.
`has_custom_header()` – Check whether a custom header is set or not.
`get_custom_header_markup()` – Retrieve the markup for a custom header.
`the_custom_header_markup()` – Print the markup for a custom header.
And a new file, `wp-includes/js/wp-custom-header.js` that handles loading videos
in custom headers.
This also enables video headers in the Twenty Seventeen and Twenty Fourteen themes.
Props davidakennedy, celloexpressions, bradyvercher, laurelfulford, joemcgill.
Fixes#38172.
Built from https://develop.svn.wordpress.org/trunk@38985
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