The `pop_until( $tag_name )` method in the stack of open elements should only be examining HTML elements, but it has only been checking the tag name. This has led to closing the wrong tags when run from inside foreign content. A very specific situation where this may arise is when a `TEMPLATE` closer is found inside foreign content, inside another template.
{{{
HTML:template SVG:template HTML:/template
<template><svg><template><foreignObject><div></template><div>
╰──< this outer TEMPLATE is closed by this one >───╯
}}}
This patch constains the method to checking for elements matching the tag name which are in the HTML namespace so that the proper detection occurs.
Developed in https://github.com/WordPress/wordpress-develop/pull/7286
Discussed in https://core.trac.wordpress.org/ticket/61576
Follow-up to [58867].
Props dmsnell, jonsurrell.
See #61576.
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