Recent Posts Widget: Use ob_end_flush() instead of ob_flush().

Merges [28195] to the 3.9 branch.

props m_i_n.
fixes #28009.

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Force users to choose a nav menu in the custom nav menu widget, for a better customizer UX

Before, they had to make a dummy change to get it to render. Now they
are made to choose a nav menu from the dropdown, which feels more
natural.

Merges [28197] to the 3.9 branch.

fixes #27878. props westonruter
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Editor: Shrink the font size for the chosen format so it fits in more locales.

Merges [28180] to the 3.9 branch.

props SergeyBiryukov.
fixes #27903.

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Ensure the editor height is saved properly as an integer.

Merges [28193] to the 3.9 branch.

props jupiterwise.
fixes #28000.

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Fix cursor style for the resize handle of the editor.

Merges [28186] for the 3.9 branch.

props avryl.
fixes #27595.

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Don't allow disabled input styling to get lost in the cascade.

Merges [28179] to the 3.9 branch.

props ocean90.
fixes #27906.

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Run WP_Editors::enqueue_scripts() on admin_print_footer_scripts, instead of admin_footer.

Fixes incompatibility with the customizer.

Merges [28187] to the 3.9 branch.

props feedmeastraycat.
fixes #27853.

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Simplify the admin menu notification selector so color schemes don't get stomped. 

Merges [28177] to the 3.9 branch.

props celloexpressions.
fixes #27920.

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Use '&mdash; Select &mdash;' instead of '-- Select --' for nav menus widget. Looks nicer.

Partially merges [28205] to the 3.9 branch.

see #27878
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