Fix styling for plugin update details Thickbox.

see #26952.
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Plupload wrapper: Call init callback on PostInit event.
Fixes the background image uploader in Customizer, which was broken after [27316].

fixes #27620.
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TinyMCE: improve setting disabled/enabled/activated states on the Unlink button, fixes #27309
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Database: Fall back from ext/mysqli to ext/mysql if the connection fails.

This allows us to avoid breaking a site that works under ext/mysql but is misconfigured for ext/mysqli.

props pento.
see #21663.

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Themes: Hide 'Add New' with no JS. see #27055.

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Theme Installer: Fix sorting, counts, keyboard navigation; add prev/next to previews.

props matveb.
see #27055.

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Fix typo in verify_file_md5() description. see [25541].

props siobhan.
see #27651.
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Clarify tooltips on Edit Image screen.

props siobhan.
fixes #27651.
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Theme Installer: Center spinner, remove plural string.

fixes #27581. see #27055.

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TinyMCE: better calculation for editor height when switching Text to Visual and back. Add stopping of editor resizing when the mouse leaves the browser window. See #27279
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Edit image modal:
- Make the calculation of the aspect ratio more robust.
- Better getting of the image height and width.
Props gcorne, see #27366
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Remove a blank line and @global reference in the PHPDoc for `comment_author_email_link()`.

Props norcross, ocean90.
Fixes #27655.

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TinyMCE: add image backgrounds for more and nextpage tags (also jshint fix for theme.js). Fixes #27106
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