directory/index.md creates the directory/ URL.

Author: Fil

src/render.ts

@@ -149,7 +149,7 @@ ${parseResult.html}</main>
 function renderListItem(p: Page, path: string): string {
   return `<li class="observablehq-link${
     p.path === path ? " observablehq-link-active" : ""
-  }"><a href="${escapeDoubleQuoted(relativeUrl(path, p.path.replace(/\/index$/, "") || "/"))}">${escapeData(
+  }"><a href="${escapeDoubleQuoted(relativeUrl(path, p.path.replace(/\/index$/, "/") || "/"))}">${escapeData(
     p.name
   )}</a></li>`;
 }

test/output/build/multi/index.html

@@ -28,7 +28,7 @@
 <input id="observablehq-sidebar-toggle" type="checkbox">
 <nav id="observablehq-sidebar">
   <ol>
-    <li class="observablehq-link"><a href="./subsection">Sub-Section</a></li>
+    <li class="observablehq-link"><a href="./subsection/">Sub-Section</a></li>
     <li class="observablehq-link observablehq-link-active"><a href="./">Multi test</a></li>
   </ol>
 </nav>

test/output/build/multi/subsection/index.html

@@ -15,7 +15,7 @@
 <input id="observablehq-sidebar-toggle" type="checkbox">
 <nav id="observablehq-sidebar">
   <ol>
-    <li class="observablehq-link observablehq-link-active"><a href="../subsection">Sub-Section</a></li>
+    <li class="observablehq-link observablehq-link-active"><a href="./">Sub-Section</a></li>
     <li class="observablehq-link"><a href="../">Multi test</a></li>
   </ol>
 </nav>