Let a header that paints its own surface show through while sticky

A section in the header set to #ffffff00 for the scrolled page still looked
white: the sticky header element paints its own white surface and a shadow
line underneath, so a transparent section showed that surface instead of the
page, and the line could be neither recoloured nor removed.

When a top-level header section defines a background of its own - normal,
hover or scrolled - the header element now gets `has-own-surface` and paints
nothing. A header without its own design keeps the white surface, so it does
not turn see-through over the content. A text colour alone is not a surface,
and a boxed column is not the whole header.

The check is a Twig function rather than a controller variable because four
paths render the header: page, archive, editor preview and area preview.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/CmsTest.php

@@ -810,6 +810,29 @@ final class CmsTest extends TestCase
         self::assertStringContainsString('class="site-header is-sticky"', $this->request('/')->getContent());
     }
 
+    /**
+     * Ein Kopfbereich mit eigener Fläche bekommt keine zweite dahinter.
+     *
+     * Beim Nachbau der Tagespflege war die Sektion beim Scrollen auf #ffffff00 gestellt und blieb
+     * trotzdem weiß: der klebende Kopf malte seine eigene Fläche und Schattenlinie darunter.
+     */
+    public function testAHeaderThatPaintsItsOwnSurfaceGetsNoSecondOne(): void
+    {
+        $settings = $this->site->load();
+        $this->site->save([...$settings, 'stickyHeader' => true, 'sameAs' => '', 'consentGroups' => $settings['consentGroups']]);
+        self::assertStringContainsString('class="site-header is-sticky"', $this->request('/')->getContent(), 'Ohne eigene Fläche bleibt die weiße des Kopfes.');
+
+        self::assertTrue(\App\Twig\HeaderExtension::ownSurface(['blocks' => [['id' => 's', 'type' => 'section', 'style' => ['desktop' => ['scrolled' => ['backgroundColor' => '#ffffff00']]]]]]));
+        self::assertTrue(\App\Twig\HeaderExtension::ownSurface(['blocks' => [['id' => 's', 'type' => 'section', 'style' => ['mobile' => ['backgroundColor' => '#294655']]]]]));
+        // Eine Textfarbe ist keine Fläche, und ein Kasten in einer Spalte macht noch keinen Kopf.
+        self::assertFalse(\App\Twig\HeaderExtension::ownSurface(['blocks' => [['id' => 's', 'type' => 'section', 'style' => ['desktop' => ['color' => '#294655']]]]]));
+        self::assertFalse(\App\Twig\HeaderExtension::ownSurface(['blocks' => [['id' => 's', 'type' => 'section', 'children' => [['id' => 'c', 'type' => 'column', 'style' => ['desktop' => ['backgroundColor' => '#ffffff']]]]]]]));
+        self::assertFalse(\App\Twig\HeaderExtension::ownSurface(null));
+
+        $css = (string) file_get_contents(__DIR__.'/../public/assets/site.css');
+        self::assertStringContainsString('.site-header.is-sticky.has-own-surface{background:transparent;box-shadow:none}', $css);
+    }
+
     /** @return iterable<string, array{string, bool}> */
     public static function imageLinks(): iterable
     {