HTML API: Respect enqueued class updates when listing attribute names

[sirreal]

get_attribute_names_with_prefix() can disagree with get_updated_html() about the class attribute when add_class() or remove_class() follows an enqueued set_attribute( 'class', ... ) or remove_attribute( 'class' ) on the same tag. This is in released WordPress since r62960 (7.1.0), which made the method read enqueued updates.

$p = new WP_HTML_Tag_Processor( '<div class="a">' );
$p->next_tag();
$p->remove_attribute( 'class' ); // Working input: $p->remove_class( 'a' );
$p->add_class( 'b' );
var_dump( $p->get_attribute_names_with_prefix( '' ), $p->get_updated_html() );

	remove_class( 'a' )	remove_attribute( 'class' )
get_attribute_names_with_prefix( '' )	array( 'class' )	array()
get_updated_html()	<div class="b">	<div class="b">

The reverse also happens: on <div>, set_attribute( 'class', 'a' ) then remove_class( 'a' ) reports class, and the output has no class attribute.

The method flushes class name updates only when they could add or remove the class attribute: a removal when the tag has the attribute, an addition when it does not. It decided whether the tag has the attribute from the input document alone. It now uses the enqueued class attribute update when one exists, which is the value class_name_updates_to_attributes_updates() starts from.

Every sequence of up to three calls to add_class(), remove_class(), remove_attribute( 'class' ) and set_attribute( 'class', ... ), on tags with no, an empty, a boolean, a single and a duplicated class attribute, was compared with a new Tag Processor's reading of get_updated_html(). On trunk 254 of 5,550 sequences disagree; with this change none do, and none of 55,550 sequences of up to four calls. get_attribute( 'class' ) agrees in every sequence on both. The new data provider test has a case for each kind of mismatch.

Trac ticket: https://core.trac.wordpress.org/ticket/65967

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Diagnosis, the sequence comparison, tests, implementation and description drafting.

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[github-actions]

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