chore: Sync the vendored UI with upstream antora-ui-default, part 1: clean ports

ui/NOTICE

@@ -3,6 +3,12 @@ This UI is a fork of the Antora Default UI
 Mozilla Public License 2.0 (see LICENSE). Modifications are
 (c) Stackable GmbH and remain under MPL-2.0.
 
+Upstream commits are periodically ported into this fork with their
+original authorship preserved in the git history. The upstream authors
+are not signatories of the Stackable CLA; their code is included solely
+under the terms of the MPL-2.0. See README.adoc, section "Provenance
+and the Stackable CLA".
+
 This directory ships with (and its build output bundles) the following
 third-party components. Build-time minification strips embedded license
 header comments; this file preserves the attributions and is included

ui/README.adoc

@@ -92,6 +92,23 @@ Copyright (C) 2017-present OpenDevise Inc. and the Antora Project.
 Use of this software is granted under the terms of the https://www.mozilla.org/en-US/MPL/2.0/[Mozilla Public License Version 2.0] (MPL-2.0).
 See link:LICENSE[] to find the full license text.
 
+=== Provenance and the Stackable CLA
+
+This directory is a fork of the MPL-2.0-licensed https://gitlab.com/antora/antora-ui-default[Antora Default UI], and upstream commits are periodically ported into it (see link:UPSTREAM[]).
+Ported commits keep their original upstream authorship in the git history.
+
+The upstream authors have not signed the Stackable CLA and are not expected to.
+Their code is included solely under the terms of the MPL-2.0 (see link:LICENSE[]), not under the CLA.
+This is intentional and applies only to the contents of this directory.
+As a practical consequence, the CLA check fails on pull requests that port upstream commits; such PRs are merged with an administrative override after review.
+
+To list the authors of ported code:
+
+[source,bash]
+----
+git log --format='%an <%ae>' -- ui/ | sort -u
+----
+
 == Authors
 
 Development of Antora is led and sponsored by {url-opendevise}[OpenDevise Inc].

ui/src/css/base.css

@@ -8,6 +8,7 @@ html {
   box-sizing: border-box;
   font-size: var(--body-font-size);
   height: 100%;
+  scroll-behavior: smooth;
 }
 
 @media screen and (min-width: 1024px) {
@@ -22,6 +23,7 @@ body {
   font-family: var(--body-font-family);
   line-height: var(--body-line-height);
   margin: 0;
+  tab-size: 4;
   word-wrap: anywhere; /* aka overflow-wrap; used when hyphens are disabled or aren't sufficient */
 }
 
@@ -98,6 +100,15 @@ object[type="image/svg+xml"]:not([width]) {
   width: fit-content;
 }
 
+.hide,
+.is-hidden {
+  display: none !important;
+}
+
+::placeholder {
+  opacity: 0.5;
+}
+
 @supports (scrollbar-width: thin) {
   body * {
     scrollbar-width: thin;

ui/src/css/breadcrumbs.css

@@ -2,7 +2,7 @@
   display: none;
   flex: 1 1;
   padding: 0 0.5rem 0 0.75rem;
-  line-height: var(--nav-line-height);
+  line-height: 1.25;
 }
 
 @media screen and (min-width: 1024px) {