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+---
+title: "Code listings with nice line wrapping and line numbers from plain CSS"
+date: 2025-07-23T23:42:00+01:00
+summary: >
+    Code listings in web pages are often a bit of a pain to use. Usually, they don't wrap on small screens. Also,
+    copy-pasting code from a code listing often copies the line numbers along with the code. Finally, many
+    implementations use heavyweight HTML and/or javascript, making them slow to render. For this blog, I wrote a little
+    CSS hack that renders nice, wrapping code blocks with line continuation markers in plain CSS without any JS.
+---
+
+Code listings in web pages are often a bit of a pain to use. Often, they don't wrap on small screens. Also, copy-pasting
+code from a code listing often copies the line numbers along with the code. Finally, many implementations use
+heavyweight HTML and/or javascript, making them slow to render (looking at you, gitlab).
+
+For this blog, I wrote an implementation that renders HTML code listings entirely without JavaScript, renders line
+numbers using plain CSS such that they don't get selected with the code, and that works with the browser to wrap in a
+natural way while still supporting the little line continuation arrows that are used to show that a line was soft
+wrapped in text editors.
+
+This blog is rendered as a static site using Hugo_ from a pile of RestructuredText_ documents. RestructuredText renders
+code listings using Pygments_ by default. Pygments hard-bakes the line numbers into the generated HTML, so I am using a
+`monkey-patched`_ hook that changes the line number rendering to just a bunch of empty ``<span>`` elements. The resulting
+HTML for a code block then looks like this:
+
+.. code:: html
+    
+    <pre class="code [language] literal-block">
+        <span class="lineno"></span>
+        <span class="line">
+            <span class="[syntax highlight token]">The </span><span class="[other syntax highlight token]">code!<span>
+        </span>
+        <!-- ... repeat once for each source line. -->
+    </pre>
+
+You can find the (rather short) source of the ``rst2html`` wrapper `below <rst2html_wrapper>`_.
+
+The CSS
+-------
+
+This modified HTML structure of the code listing gets accompanied by some CSS to make it flow nicely. Here is a listing
+of the complete CSS controlling the listing. The only bit that isn't included here is the actual syntax styling rules
+for the pygments tokens.
+
+.. code:: css
+
+    /*****************************************************/
+    /* Code block formatting / syntax highlighting rules */
+    /*****************************************************/
+
+    .code {
+        font-family: "Fira Code";
+        font-size: 13px;
+        text-align: left; /* Override default content "justify" alignment */
+        white-space: pre-wrap;
+        word-wrap: break-word;
+        overflow-x: auto;
+        display: grid;
+        align-items: start;
+        grid-template-columns: min-content 1fr;
+    }
+
+    .code > .line {
+        padding-left: calc(2em + 5px);
+        text-indent: -2em;
+        padding-top: 2px;
+        min-width: 15em;
+    }
+
+    /* Make individual syntax tokens wrap anywhere */
+    .code > .line > span {
+        overflow-wrap: anywhere;
+        white-space: pre-wrap;
+    }
+
+    /* We render line numbers in CSS! */
+    .code > .lineno {
+        counter-increment: lineno;
+        word-break: keep-all;
+        margin: 0;
+        padding-left: 15px;
+        padding-right: 5px;
+        overflow: clip;
+        position: relative;
+        text-align: right;
+        color: var(--c-text-muted);
+        border-right: 1px solid var(--c-fg-highlight);
+        align-self: stretch;
+    }
+
+    /* We also handle line continuation markers in CSS. */
+    .code > .lineno::after {
+        position: absolute;
+        right: 5px;
+        content: "\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳";
+        white-space: pre;
+        color: var(--c-text-muted);
+    }
+
+    /* Insert the actual line number */
+    .code > .lineno::before {
+        content: counter(lineno);
+    }
+
+    .code::before {
+        counter-reset: lineno;
+    }
+
+    footer {
+        margin-top: 30px;
+        align-self: center;
+        margin-bottom: 10px;
+        text-align: center;
+        padding: 5px 25px 5px 25px;
+    }
+
+    .code .hll {}
+    /* Following are about 50 lines that define the styling of each kind of pygments syntax highlight token. These lines
+       all look like the following: */
+    .code .c   { color: var(--c-text); font-weight: 400 } /* Comment */
+
+This CSS does a few things:
+
+ 1. It renders the ``<pre>`` code listing element using a two-column CSS ``display: grid`` layout. The left column is
+    used for the line numbers, and the right column is used for the code lines.
+ 2. It numbers the lines using a `CSS Counter`_. CSS counters are meant for things like numbering headings and such, but
+    they are a perfect fit for our purpose.
+ 3. It inserts the counter value as the line number into the ``<span class="lineno">`` element's ``::before``
+    pseudo-element. A side effect of using the ``::before`` pseudo-element is that without doing anything extra, the
+    line numbers will remain outside of the normal text selection so they will neither be highlighted when selecting
+    listing content, nor will they be copied when copy/pasting the listing content.
+ 4. It inserts a string of ``"\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳"`` into the line number span's
+    ``::after`` pseudo-element. This string evaluates to a sequence of unicode arrows separated by line breaks, and
+    starting with an empty line. The ``::after`` pseudo-element is positioned using ``position: absolute``, and the
+    parent ``<span class="lineno">`` has ``position: relative`` set. This way, the arrow pseudo-element gets placed on
+    top of the lineno span without affecting the layout at all. By setting ``overflow: clip`` on the parent ``<span
+    class="lineno">``, the arrow pseudo-element gets cut off vertically wherever the parent lineno element naturally
+    ends.
+
+The line number span is inserted into the parent ``<pre>`` element's CSS grid using ``align-self: stretch``, which
+causes it to vertically stretch to fill the available space. Since the line number span only contains the line number,
+its minimum height is a single line. As a result, it will stretch higher only when the corresponding code line in the
+right grid column stretches vertically because of line wrapping. When that happens, part of the arrow pseudo-element
+starts showing through from behind the ``overflow: clip`` of the line number span, and one arrow gets rendered for each
+wrapped listing line.
+
+When the page is too narrow, we don't want the code listing's lines to wrapp into a column of single characters. To
+prevent that, we simply set a ``min-width`` on the ``<span class="line">`` in the right column, and set ``overflow-x:
+auto`` on the listing ``<pre>``. This results in a horizontal scroll bar appearing whenever the listing gets too narrow.
+
+You can try out the line wrapping by resizing this page!
+
+rst2html wrapper
+----------------
+
+Here is the python ``rst2html`` wrapper that monkey-patches code rendering. I made hugo invoke this while building the
+page by simply overriding the ``PATH`` environment variable.
+
+.. code:: python
+
+    #!/usr/bin/env python3
+    # Based on https://gist.github.com/mastbaum/2655700 for the basic plugin scaffolding
+
+    import sys
+    import re
+
+    import docutils.core
+    from docutils.transforms import Transform
+    from docutils.nodes import TextElement, Inline, Text
+    from docutils.parsers.rst import Directive, directives
+    from docutils.writers.html4css1 import Writer, HTMLTranslator
+
+
+    class UnfuckedHTMLTranslator(HTMLTranslator):
+        def __init__(self, *args, **kwargs):
+            super().__init__(*args, **kwargs)
+            self.in_literal_block = False
+
+        def visit_literal_block(self, node):
+            # Insert an empty "lineno" span before each line. We insert the line numbers using pure CSS in a ::before
+            # pseudo-element. This has the added advantage that the line numbers don't get included in text selection.
+            # These line number spans are also used to show line continuation markers when a line is wrapped.
+            self.in_literal_block = True
+            self.body.append(self.starttag(node, 'pre', CLASS='literal-block'))
+            self.body.append('<span class="lineno"></span><span class="line">')
+
+        def depart_literal_block(self, node):
+            self.in_literal_block = False
+            self.body.append('\n</span></pre>\n')
+
+        def visit_Text(self, node):
+            if self.in_literal_block:
+                for match in re.finditer('([^\n]*)(\n|$)', node.astext()):
+                    text, end = match.groups()
+
+                    if text:
+                        super().visit_Text(Text(text))
+
+                    if end == '\n':
+                        if isinstance(node.parent, Inline):
+                            self.depart_inline(node.parent)
+                        self.body.append(f'</span>\n<span class="lineno"></span><span class="line">')
+                        if isinstance(node.parent, Inline):
+                            self.visit_inline(node.parent)
+
+            else:
+                super().visit_Text(node)
+
+
+    html_writer = Writer()
+    html_writer.translator_class = UnfuckedHTMLTranslator
+    docutils.core.publish_cmdline(writer=html_writer)
+
+.. _Hugo: https://gohugo.io/
+.. _RestructuredText: https://www.sphinx-doc.org/en/master/usage/restructuredtext/index.html
+.. _Pygments: https://pygments.org/
+.. _`monkey-patched`: https://en.wikipedia.org/wiki/Monkey_patch
+.. _`CSS Counter`: https://developer.mozilla.org/en-US/docs/Web/CSS/counter