'Convert line breaks into HTML' prepends any <input> with <br>

Changes are made on on 11.x (our main development branch) first, and are then back ported as needed according to the Core change policies → . Also mentioned on the version → section of the list of issue fields documentation.

I was able to reproduce the issue by following these steps:

1. Enabled the "Convert line breaks into HTML" filter for a text format at /admin/config/content/formats.
2. Created a node using that text format.
3. Entered Test in the body using the "Source" view in CKEditor, ensuring no extra whitespace or newlines were present.
4. Saved the node and viewed the output on the frontend.

Even though the saved source remains

Test

, the rendered output includes a line break (
) between "Test" and the tag.

_filter_autop() function in filter.module is responsible for converting line breaks into
and wrapping text in

tags. Within this function, there's a $block regex pattern that defines all block-level elements:
$block = '(?:table|thead|tfoot|caption|col|colgroup|tbody|tr|td|th|div|dl|dd|dt|ul|ol|li|pre|select|option|form|map|area|blockquote|address|math|input|p|h[1-6]|fieldset|legend|hr|article|aside|details|figcaption|figure|footer|header|hgroup|menu|nav|section|summary)';
Since is listed as a block-level element,a newline is added before any tag. Later in the processing, this newline gets transformed into

Proposed Resolution:
According to HTML specifications, is an inline-level element, not a block-level one. Therefore, it should not be included in the $block list used by _filter_autop().

Removing input from the $block list resolves the issue, as the newline is no longer added before the tag, and no
is inserted.