fix(tags): write tag pages into tags/ even next to a Tags.md page

Source/Mdu.Test/TagsTest.cs

@@ -170,6 +170,24 @@ public void WithoutVocabularyTheMostUsedSpellingNamesTheTagAndMduWritesTheOvervi
         });
     }
 
+    [Fact]
+    public void TagFolderKeepsItsCasingNextToTheVocabularyPageTags()
+    {
+        WithSite(StandardSite(), root =>
+        {
+            var site = Path.Combine(root, "site");
+            // A folder left from an earlier render, as the redirect of Tags.md creates it on a case-insensitive disk.
+            Directory.CreateDirectory(Path.Combine(site, "Tags"));
+            var (result, errors) = Render(root);
+            Assert.True(result == 0, errors);
+
+            Assert.Contains("tags", Directory.EnumerateDirectories(site).Select(Path.GetFileName), StringComparer.Ordinal);
+            Assert.True(File.Exists(Path.Combine(site, "tags", "security.html")));
+            var (check, checkErrors) = Capture(() => CheckSiteCommand.Execute(new DirectoryInfo(site), []));
+            Assert.True(check == 0, checkErrors);
+        });
+    }
+
     [Fact]
     public void DocumentationFolderNamedTagsCollidesOnlyWhenTheSiteUsesTags()
     {

Source/Mdu/Service/Visitors/MarkdigVisitor.Tags.cs

@@ -27,7 +27,7 @@ public IReadOnlyList<string> WriteTagPages()
     {
         if (_siteTags.Tags.Count == 0) return [];
         var folder = Path.Combine(_outPath, SiteTags.Folder);
-        Directory.CreateDirectory(folder);
+        CreateTagFolder(folder);
         var written = new List<string>();
         var anchorPage = _siteTags.VocabularyPage ?? _navigationRoots.FirstOrDefault();
         var indexPath = Path.Combine(folder, "index.html");
@@ -80,6 +80,27 @@ private string ExpandTagLists(MarkdownFile page, string markdown, string outputD
             GetRelativeUrl(segments, [SiteTags.Folder, tag.Slug + ".html"]), tag.Pages.Count)).ToArray();
     }
 
+    /// <summary>
+    /// Creates <c>tags/</c> with exactly this casing. On a case-insensitive file system the redirect <c>Tags/index.html</c>
+    /// of a vocabulary page <c>Tags.md</c>, or an earlier render, may already have created the folder as <c>Tags/</c>;
+    /// links to <c>tags/…</c> would then break on a case-sensitive web server, so the folder is renamed.
+    /// </summary>
+    private static void CreateTagFolder(string folder)
+    {
+        var parent = Path.GetDirectoryName(folder)!;
+        var name = Path.GetFileName(folder);
+        if (Directory.Exists(folder) && Directory.EnumerateDirectories(parent).Select(Path.GetFileName).ToArray() is var names &&
+            !names.Contains(name, StringComparer.Ordinal) &&
+            names.FirstOrDefault(entry => string.Equals(entry, name, StringComparison.OrdinalIgnoreCase)) is { } existing)
+        {
+            // A case-only rename goes through a temporary name, which every file system accepts.
+            var temporary = Path.Combine(parent, name + "-" + Guid.NewGuid().ToString("N"));
+            Directory.Move(Path.Combine(parent, existing), temporary);
+            Directory.Move(temporary, folder);
+        }
+        Directory.CreateDirectory(folder);
+    }
+
     private bool IsInTagFolder(string outputPath) =>
         Path.GetFullPath(outputPath).StartsWith(
             Path.GetFullPath(Path.Combine(_outPath, SiteTags.Folder)) + Path.DirectorySeparatorChar, PathComparison);