Re-instantiate the `MediaElementPlayer` instance when switching mime types in playlists.

Fixes #27608.


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Forward pingback IP during pingback verification.

props tellyworth, nacin.
fixes #27613.

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Clarify inline documentation for the 'html5' case and PHPDoc return in `add_theme_support()`.

Props jond3r.
Fixes #27278.

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Clarify PHPDoc return description for `media_handle_upload()`.

Props jdgrimes.
Fixes #27519.

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Revert [27654] for strings.

props westonruter.
fixes #27485.
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Widget Customizer: Remove WidgetCustomizer.showFirstSidebarIfRequested().

It's currently unused and needs another iteration.

see #27290.
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Widget Customizer: Use postMessage to highlight widgets in preview or sections/controls in Customizer.

fixes #27622.
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Cleanup [27892].
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Make `<audio>` elements in playlists responsive.

See #27625.


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Call MediaElement's shim method for `setSrc()` when dynamically updating the playing source in playlists, which will properly set the source for mobile.

Make the labels for playlist items slightly smaller to account for size on mobile.

Fixes #27625.


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Theme Installer fixes:
 * Better more filters section. props sonjanyc for some mockups.
 * Better handling of no results.
 * Use # for hrefs.

props matveb.
see #27055.

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TinyMCE: update to 4.0.21, see #24067
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Edit Image modal:
- Move all advanced options under a single “Show advanced options” toggle that mirrors the behavior and look-and-feel of the wplink modal.
- Switch to using <select> for the Size and Link To.
- Bring back the field for CSS Class for the image, but don’t incorporate the internally managed WordPress classes (size-, wp-image-, etc…).
- On larger screen sizes, float labels to the left. When the width drops below 900px, stack the label above the fields.
- Keep image at top on screen sizes where the two columns are stacked into a single column.
- Don't replace the nodes in the editor DOM so we don't stomp on any custom attributes that the user may have added via the Text editor or some other mechanism.
Props gcorne, see #27366
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Gallery preview: add a "No items found" state which is displayed when rendering the gallery wpview and no attachments are found. Props gcorne, fixes #27558
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wpLink: fix spinners selectors and add some throttling when searching with AJAX while the user is typing, fixes #27615
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Drag and drop files on the editor to upload: add new argument to wp_editor() to enable, fixes #27465
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Improve some inline documentation for class properties in `WP_Customize_Section`.

Props leewillis77 for the initial patch.
Fixes #26245.

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Second-pass inline documentation improvements for WP_Customize_Widgets.

See #27534.

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Coding standards and readability spacing for WP_Customize_Widgets.

Also explicitly re-mark `_capture_filter_pre_update_option()` and `_capture_filter_pre_get_option()` as public.

See #27534.

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Background Updates: Record Plugin & Theme update statistics like we for for Core updates, Pass Plugin/Theme update objects into the Background updater for consistency with Core & Translations. See #27633

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