* 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.
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