Rework post page (lead, pager), add skip link, unified focus ring, reduced motion and keyboard-accessible menu

Elternbeirat.Web/Features/Posts/PostNeighbors.cs

@@ -0,0 +1,65 @@
+using Elternbeirat.Contracts;
+
+namespace Elternbeirat.Web.Features.Posts;
+
+/// <summary>
+///     The posts next to a given post in time: the one published before it and the
+///     one published after it.
+/// </summary>
+/// <param name="Previous">
+///     The older post, or <see langword="null"/> if the post is the oldest.
+/// </param>
+/// <param name="Next">
+///     The newer post, or <see langword="null"/> if the post is the newest.
+/// </param>
+/// <remarks>
+///     "Previous" and "next" follow time, as a reader moving through the news would:
+///     "Vorheriger Beitrag" is the older one, "Nächster Beitrag" the newer one. The
+///     list order of the posts page (newest first) is the other way round, which is
+///     why the direction is pinned down here instead of being left to the markup.
+/// </remarks>
+/// <seealso cref="PostDetail"/>
+public sealed record PostNeighbors(Post? Previous, Post? Next)
+{
+    /// <summary>
+    ///     Finds the neighbours of the post with the given slug.
+    /// </summary>
+    /// <param name="posts">
+    ///     All public posts, newest first, as <c>PocketBaseClient.GetPostsAsync</c>
+    ///     returns them.
+    /// </param>
+    /// <param name="slug">The slug of the post being shown.</param>
+    /// <returns>
+    ///     The older and the newer post. Both are <see langword="null"/> if the slug is
+    ///     not in the list, or if it is the only post.
+    /// </returns>
+    /// <remarks>
+    ///     The slug is compared case-insensitively, like the route lookup in
+    ///     <see cref="PostDetail"/>, so a page reached as <c>/posts/New-Board</c> still
+    ///     gets its neighbours.
+    /// </remarks>
+    /// <example>
+    ///     <code>
+    ///         // posts = [March, February, January] (newest first)
+    ///         var neighbors = PostNeighbors.Of(posts, "february");
+    ///         // neighbors.Previous is January, neighbors.Next is March
+    ///     </code>
+    /// </example>
+    public static PostNeighbors Of(IReadOnlyList<Post> posts, string slug) =>
+        IndexOf(posts, slug) is var index and >= 0
+            ? new PostNeighbors(
+                Previous: index + 1 < posts.Count ? posts[index + 1] : null,
+                Next: index > 0 ? posts[index - 1] : null)
+            : new PostNeighbors(null, null);
+
+    /// <summary>
+    ///     The position of the post with <paramref name="slug"/>, or <c>-1</c>.
+    /// </summary>
+    private static int IndexOf(IReadOnlyList<Post> posts, string slug) =>
+        posts
+            .Select((post, index) => (post, index))
+            .FirstOrDefault(
+                entry => string.Equals(entry.post.Slug, slug, StringComparison.OrdinalIgnoreCase),
+                (null!, -1))
+            .index;
+}