wpautop incorrectly handling paragraphs within block elements

[pento]

When there are two line breaks within a block element, wpautop() will replace it with </p><p>, but the corresponding opening and closing tags won't exist.

For example, <div>a\n\nb</div> will produce <div>a</p><p>b</div>, when it really should produce <div>\n<p>a</p>\n<p>b</p>\n</div>.

[pento]

38656.diff​ is a first pass at fixing this, it needs many more unit tests.

[pento]

38656.2.diff​ makes this all work with nested peeable and unpeeable elements, and adds more unit tests.

[pento]

38656.3.diff​ adds code comments, and some more complex tests.

There's probably value in adding some intelligence to the closing HTML tag detection, so we can handle some basic invalid HTML. For example, a browser will automatically correct <div><span>foo</div>, and treat it as <div><span>foo</span></div>, but wpautop() will get $peeable out of sync, causing problems.

[desrosj]

I was able to reproduce this bug on trunk. 38656.4.diff​ is a refresh to apply cleanly after coding standards changes.

Running the unit tests using --group 38656 passes, but there is a failure elsewhere in test_that_wpautop_doses_not_add_extra_closing_p_in_figure() when running --group formatting.

When testing in the editor, I see the This block contains unexpected or invalid content message when switching back to visual editor after entering the test markup in the description.