Unexpected markup in page title source

We hit this same bug building the UI Suite UIkit theme: on 1.0.0-beta7 / Drupal 11.4.7-dev, a node title themed with a wrapper prints the literal escaped tags in the page layout
instead of rendering. Matches the MarkupInterface (string) cast in PageLayoutPageVariant::replaceTitleAndContent() described here.

We worked around it in our theme with a page_title preprocess. We'll test MR !386 against our reproduction rather than open a duplicate issue - thanks to the reporter and to !386's author.

AI-Generated: Yes

Indeed, something wrong happened in #3618072: Placeholders and real render, in Canvas and Preview where this prop source has been changed to a slot source:

id: 'page_title',
label: new TranslatableMarkup('[Page] Title'),
description: new TranslatableMarkup('The Drupal Page title block (page_title).'),
- prop_types: ['string'],
+ prop_types: ['string', 'slot'],
tags: [],

This change is rendering PageTitleSource with the legacy page-title.html.twig, so hardcoded away the control of the display builder, which is exactly what we are fighting against by proposing Display Builder and the modernization of Drupal theming.

I am very sorry about such a situation. I will rebase first.

Rajab, is it still OK for you?

I agree to the last update on the MR
---
We are moving to having all dynamic - better not to keep some parts as static. even the default core title.

What I see that a Theme builder will decide to have a full HTMX support and then a full support for Display Builder.
As I'm testing that in UI Suite UIkit and Webtheme, Website and with some other setups
and a Web Dashboard too - with Display Builder

I agree to the last update on the MR

So, OK for RTBC (except the little pipeline fail I ll fix before, of course)?

@pdureau I pushed a fix to pipeline, please check.

Thanks. Will check