fix(renderer): drop the indent an indented fence close drags into code content

Copilot review on PR #804:
- An indented closing fence line ("  ```") left its leading spaces as a
  trailing whitespace-only line inside the rendered code block: the content
  capture runs up to the backtick run and the close-line indent precedes it, so
  it was captured as content. Strip a trailing newline PLUS any trailing indent
  (/\n[ \t]*$/ instead of /\n$/); a column-0 close is unaffected. Red-green
  pinned (content is exactly "  x = 1", no trailing whitespace line).
- Correct a stale test docstring claiming the fence open anchor allows "up to 3
  spaces" of indent — it allows arbitrary indent (the 4-space case is pinned
  separately).

(cherry picked from commit e5e48a788a)

tests/test_renderer_js.py

@@ -1710,15 +1710,28 @@ def test_blockquoted_fence_renders_as_code() -> None:
 
 
 def test_indented_fence_still_renders_as_code() -> None:
-    """The open anchor allows up to 3 spaces of indentation (CommonMark), so a
-    legitimately indented fence (e.g. under a list item) still renders as code
-    rather than as a paragraph of literal backticks.  A bare ``^`` anchor
-    would have dropped it."""
+    """The open anchor allows arbitrary leading indent, so a legitimately
+    indented fence (e.g. under a list item) still renders as code rather than a
+    paragraph of literal backticks.  (A bare ``^`` anchor would drop it; the
+    deeper 4-space-indent case is pinned separately.)"""
     out = _render("  ```py\n  x = 1\n  ```")
     assert "<pre><code" in out, "indented fence dropped (not rendered as code):\n" + out
     assert "x = 1" in out
 
 
+def test_indented_fence_close_leaves_no_trailing_whitespace_line() -> None:
+    """An indented closing line's leading spaces must NOT survive as a trailing
+    whitespace-only line inside the code block: the content strip removes a
+    trailing newline PLUS any indent the close dragged into the capture (a
+    `` ``` `` closed at column 0 is unaffected).  Copilot review, PR #804."""
+    out = _render("  ```py\n  x = 1\n  ```")
+    m = re.search(r"<code[^>]*>(.*?)</code>", out, re.S)
+    assert m, "no <code> block:\n" + out
+    assert m.group(1) == "  x = 1", "indented fence close left a trailing whitespace line: " + repr(
+        m.group(1)
+    )
+
+
 # ---------------------------------------------------------------------------
 # Fix 4 — <details> open anchored to line start (B5). The details pass ran
 # with an unanchored open, so a `<details>` mentioned mid-line inside inline

turnstone/shared_static/renderer.js

@@ -381,7 +381,10 @@ function _renderMarkdownBody(text) {
         "<pre><code" +
           (cssLang ? ' class="language-' + escapeHtml(cssLang) + '"' : "") +
           ">" +
-          escapeHtml(code.replace(/\n$/, "")) +
+          // Strip the trailing newline plus any whitespace an indented close
+          // dragged into the content (a `  ```` close would otherwise leave a
+          // whitespace-only last line in the code block).
+          escapeHtml(code.replace(/\n[ \t]*$/, "")) +
           "</code></pre>",
       );
       // Store the fence source WITHOUT the re-emitted indent/marker prefix so