Avoid an expensive attachment counting query on the post editing screen.

Merges [28191], [28194] to the 3.9 branch.

props johnbillion.
fixes #27985.

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Drag/drop on the editor to upload: don't trigger the uploader when selected text is being dragged from one window to another.

Merges [28189] to the 3.9 branch.

props azaozz.
fixes #27880.

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Customizer: Avoid hiding 'Remove' buttons unrelated to custom headers.

Merges [28266] to the 3.9 branch.

props rzen, ehg.
fixes #27848.

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Customizer: Only tie header button action events to the header controls.

Merges [28269] to the 3.9 branch.

props danielbachhuber.
fixes #28046. see #27848.

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Refinements for asynchronous rendering in `wp.mce.media.PlaylistView`.

Merges [28182] to the 3.9 branch.

props gcorne, wonderboymusic.
fixes #27899.

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Fix persistence of <track> elements in the body of a [video] shotcode in TinyMCE views.

Merges [28183] and [28169] to the 3.9 branch.

props azaozz, wonderboymusic.
fixes #27915.

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Custom header: Avoid warnings in the process_default_headers() method.

Merges [28275] to the 3.9 branch.

props obenland.
fixes #27850.

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Multisite: Fix case sensitivity regressions in 3.9.

Merges [28276] to the 3.9 branch.

props jeremyfelt.
fixes #27866.

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Multisite: Treat 'www' as a special subdomain, reversing 3.9 regression.

Merges [28280] to the 3.9 branch.

props jeremyfelt.
fixes #27927.

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3.9.1-RC1

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